In general, it may be said that the breathing process of mollusks does not markedly differ from the respiratory methods of higher forms of animals. Whether the breathing is effected by gills or by a pulmonary sac, the essential features of the system are the same as in fishes or mammals. There are, however, some rather peculiar developments in the breathing system of gasteropods that are worthy of notice. The gills themselves differ throughout the various orders of this class both as to their structure and their relative position; in one of the orders (the Pulmonata) the gills have entirely disappeared, and are replaced by a pulmonary sac which fulfils the purpose of a lung.
It is a generally accepted theory that all land or air-breathing mollusks were originally marine, but by a gradual change in their habits have developed into terrestrial forms, losing their gills and acquiring in their place organs which correspond to lungs. The theory is supported by the fact that many species of marine mollusks live between tide-marks, and for several hours daily are obliged to maintain life outside of their native element. The Littorinidæ, which may always be found in abundance on both the east and the west American coasts, usually live high above low-tide line, and some tropical species of this genus actually live in the trees above the reach of high tide. They have gills just as in Buccinum or Fulgur, but the Littorina shell is strongly suggestive of certain land forms, and there is reason to believe that the genus is in course of evolution into a pulmonate. On the other hand, there are to be found in fresh water numerous genera which are true pulmonates, and which are obliged to come to the surface every few minutes to obtain air. One notable genus of marine gasteropods possesses both lungs and gills, and one marine form has entirely lost its gills and breathes only by means of lungs. It is quite likely that all mollusks are able to breathe more or less all along the exposed mantle surface, which is usually [pg339] crowded with capillaries with exceedingly thin walls. This very remarkable method of surface respiration is proved to exist in the nudibranch or shell-less marine Gasteropoda, for they possess neither gills nor lungs.
Lying just under the gills, and side by side with them, is a smaller, plume-like organ having the appearance of a smaller gill; it is called the osphradium. The office of this organ is not definitely known, but the supposition is that it is the seat of the olfactory nerves. The osphradia are not always present, but whenever found they are placed in close conjunction with the gills, in order, probably, that the animal may best determine the quality of the water brought by the siphon to the breathing-organs. In Buccinum the osphradium is especially prominent. In all mollusks the sense of smell is highly developed. A dead animal anchored to the bottom will very soon attract the carnivorous Mollusca from every direction. This is not a bad way to trap certain species that live upon rocky stations and are otherwise difficult to secure by the dredge.
On the left-hand side (right of the animal), and adhering to the under side of the mantle flap, which has been thrown back, a somewhat inflated and convoluted tube, the rectum, will be seen. It terminates in an opening just within the mantle-edge. By the side of this are the renal and genital openings.
Having now examined the external organs, we may investigate
the internal anatomy of our subject. To do this thoroughly requires
considerable skill, but for the purpose of merely discovering
the relative positions of the principal internal organs a little
care only is necessary. The work will be greatly facilitated by
boiling the animal for a few minutes, for this serves to harden
the tissues and to separate the various organs in a most satisfactory
manner.
To begin, open the proboscis from the mouth, cutting back so as to expose the esophagus. It will be seen that the throat is long and very muscular. [pg340]
Hidden among the red stripes of muscular fiber which line the inner surface of the throat will be found a fleshy strip covered upon one side with many transverse rows of chitinous teeth. This is the lingual ribbon, or radula. Remove as much of it as possible, and examine it with a hand-glass. The radula is a most important organ, and is a unique possession of Mollusca, except the pelecypods, or bivalves. Usually it lies coiled up like a spiral when not in use; its purpose is to tear and rasp food.
The radula has been the subject of much study by conchologists, and has furnished the basis for an arrangement of the numerous sections of gasteropods into somewhat well-defined groups. If the observer has a strong glass he will notice that each transverse row of teeth on the ribbon consists of a median or central tooth with several points or "cusps." Fulgur has a median tooth with three cusps; Buccinum one with six. On each side of the median tooth is a lateral tooth, which in Fulgur will be seen to have six cusps, and in Buccinum five. In many patterns of radulæ there is, besides the median and lateral teeth, still another paired row, called the marginals. In the accompanying figures are given some examples which show small sections of lingual ribbons. While all the radulæ of mollusks are capable of being grouped by their general characteristics, yet in no two species of mollusks are the radulæ quite identical. [pg341]
The digestive tract will be found to widen out a short distance back into a crop or stomach, and, continuing still further, to lose itself in a soft, brownish mass within the coiled spire, which is the liver. The very large size of the liver leads one to suspect that Fulgur and Buccinum must be voracious creatures to need so large an organ for the secretion of bile. In many species of mollusks the stomach and intestines are filled quite solidly, at times, with a gelatinous transparent substance called the crystalline stylet. Just why the digestive tract should be clogged with this substance no one has yet been able to explain, so here again is a chance for original investigation. The intestine curves about after reaching the liver, and comes forward again to appear once more as the rectum, clinging to the inner surface of the free portion of the mantle.
Closely associated with the liver, but differing slightly in color, is the gonad, or organ in which the genital products are formed. Situated dorsally and forward of the liver and gonad is a large renal gland, which may readily be detected by its peculiar structure. In some forms the kidney is closely associated with the gonad, and seems to coöperate with the latter in the generative functions.
To find the heart, make an incision into the body just at the posterior end of the gills. The heart is white and round, and is inclosed within a cavity known as the pericardium; it has a ventricle and one or two auricles, although in Buccinum and Fulgur there is but one auricle.
There is nothing remarkable about the vascular system to distinguish it from that of many higher forms of animals. It is, however, not completely closed—by which is meant that the blood is not always contained within arteries or veins, and that it does sometimes flow into other organs and floods certain other body-cavities, although the vascular system of mollusks is by no means so completely open as is that of insects and crustaceans. [pg342]
Having examined the animal which inhabits the shell and gained an idea of its anatomy, we may now turn to an inspection of the house which it has built for itself, and here we shall find a delightful subject for study.
An industrious hour of collecting upon almost any beach will provide the student with an abundance of interesting shells. Putting aside the bivalves, the univalve or gasteropod shells remaining should be closely examined for the purpose of becoming acquainted with their various conchological features. The collector will see at once that he has a number of species that differ more or less widely in their general form, size, texture, and sculpture. Some shells will be long in proportion to their width, with many turns of the spire which probably terminates in a sharply pointed apex; others will be almost round, with comparatively few spiral turns which end in a blunt apex, giving to the specimen outlines suggestive of a dome. A wide range of characters will be found in the mouths or apertures of the shells, some being almost if not quite round, others oblong and with a notch cut into the lower portion of the opening, and others possibly with this notch extended into a sort of semi-inclosed channel. A first lesson in classification of the Mollusca may be taken by dividing the results of the first day's collecting of univalve shells into groups according to these prominent shell-characters.
A good knowledge of the parts of a gasteropod shell is essential to the student, and it can readily be acquired. Four specimens are given in the accompanying figures, representing highly divergent [pg343] types of marine gasteropods, and their respective characteristics may be noted by comparing them one with another: Fasciolaria tulipa (Plate LXXVI.), a Floridian shell; Polynices heros, a common species found on all the sandy shores of the American Atlantic coast; Murex tenuispina, an Indo-Pacific species much prized by collectors; and Acmæa testudinalis, commonly found in Maine and Massachusetts at low tide clinging to rocks.
The extreme top of the shell is called the apex, and it may be either sharply pointed or obtuse. Each turn is called a whorl; the last one, and always the largest, is called the body-whorl, the others collectively forming the spire of the shell. When the animal first emerges from the egg it is always provided with one or two whorls, which generally may be distinguished at the apex as smoother than the rest of the spire, and a faint line may be discovered where the shell began its growth after birth. These [pg344] first whorls of embryonic growth are usually referred to as the nepionic whorls. The interstices between the successive whorls are called sutures, and these lines of juncture may be shallow or deep according to the convexity of the whorls. The sculpturing of the whorls is often of great complexity, and exhibits nodules, varices, ribs, revolving striæ, spinous processes, and lines of growth. It frequently happens that there is a complete absence of any sculpturing whatever, the whole surface of the shell being perfectly smooth, save for the faint lines of growth to be detected only under a magnifying-glass. The outer edge of the mouth or aperture of the shell is called the outer lip. This may be more or less thickened, expanded, or simple. The inner lip is called the columella, and may be greatly extended anteriorly. When the mouth of the shell is round, and there is no notch or indentation either above or below, it is said to be continuous.
The anterior canal is for the accommodation of the siphon of the animal, which, as noted above, may be very long or almost entirely absent; therefore the character of the shell generally indicates whether or not the animal itself possesses a long siphon. There is in some forms a notch at the upper extremity of the mouth, which is for the accommodation of the anal discharges. When present it is called the posterior canal.
It will be noted that all the specimens figured as examples, save one, are spiral, the coils revolving about an imaginary axis. When the whorls are not too closely coiled there is left open to the view a hollow space throughout the entire length of the shell, which is called the umbilicus. When the coils of the shell are very compact there is no umbilicus to be seen. The umbilical opening may be very large, giving a clear view of the inner surfaces of all the whorls right through to the apex. The usual gasteropod shell is in reality a hollow tube wound about an imaginary axis. Since the tube increases in size as it progresses, the base of the shell is necessarily wider and larger than the apex. Thus all gasteropod shells may be reduced in shape to the single type of a cone or top, modified by the excessive length or shortness of the spire, or otherwise altered by special peculiarities. In Polynices duplicata (page 368), it will be observed that while the [pg345] umbilicus is fairly large, its external opening, in the adult shell, has been almost completely closed by a callous process thrown off from the inner or columellar lip.
Gasteropod shells may be sinistral or dextral, according as the whorls turn to the left or right. The great majority of marine species are dextral, having the opening on the right, although a few species exhibit the curious property of being constructed upon either the one or the other plan without apparent reason. Sinistral specimens of many normally dextral species have been discovered, but they are so rare as to warrant the belief that such specimens are merely deformities or monstrosities resulting from some accident of birth.
The collector will soon learn to distinguish between young and adult specimens, for those finishing touches of wonder and beauty which the mollusk gives to his protective covering are made only by the adult. The outer lip of young gasteropod shells is usually thin and fragile; even if slightly thickened, it has an unformed or unfinished appearance.
Shells differ greatly in their structure, many species being porcelanous like china, others glassy, and many more are of a softer chalky composition. In the latter case the shell is usually covered with a thick membranous skin, which, when removed, leaves a dull, lusterless, white body beneath. Shells of this texture, when cast upon the beach, soon yield to the weathering action of sun and air. The porcelanous shells are composed of successive layers of carbonate of lime, throughout which is a filmy membranous framework of a substance similar to that of which the covering of the Crustacea is formed. The mode of deposition of the various layers of crystalline calcic carbonate and the peculiar lineation of their outer surfaces give rise to the iridescent or nacreous appearance of some shells.
With the exception of those species in which the mantle of the animal is extended over the edge of the aperture and more or less envelops the entire test, there is an outer skin of membranous or organic matter which overlies the surface of the shell. This skin is sometimes quite thick, often hairy, and is usually of a dull greenish- or brownish-yellow color. In many genera this [pg346] outer covering of the shell is excessively thin. In old specimens the epidermis is usually worn away from the apical whorls. It often happens that the shells of the most exquisite colors and markings are thus rendered somber and unattractive by their tenants during life; when the animal dies, or when such specimens are kept in a cabinet for a period of time, the epidermis dries, cracks, and falls off, revealing the wealth of color and design beneath.
The growth of the gasteropod shell is accomplished by the exudation from the margin of the animal's mantle of a liquid containing the shelly matter in solution. The mantle-edge is provided with a complicated system of glands and pores, from which is secreted this "stony liquor." This more or less viscous liquid, containing the carbonate of lime and the other inorganic materials of which the shell is composed, hardens upon exposure, and the shelly matter is then deposited in crystalline form around the edges or lip of the shell aperture. The gasteropod shell therefore grows by the continual building out of its aperture through successive depositions of shelly matter at the extreme edge of the lip. At the extreme edge of the mantle margin are situated those glands which secrete the materials for the epidermis of the shell, and as one would therefore expect, this outermost layer of epidermis is first produced in the advancing growth of the shell. There also are situated the pigment-glands, which produce the color-secretions. The various layers of the shelly substance are successively deposited inside the mouth of the shell by glands situated just back of the extreme edge of the mantle margin. Thus in the growth stage, if one could examine closely the aperture of a gasteropod shell, one would observe at the extreme tip of the lip this projecting epidermis, just beneath it and just inside the aperture a thin deposit of shelly matter, just beneath this, and farther in, another layer, and still farther in a third layer.
The growth of nearly all gasteropod shells is marked by periods of rest. During the inactive seasons the creature may thicken the edge of the aperture to a greater or less extent by an extra deposit of shelly matter, for otherwise the thin lip might soon be [pg347] injured or broken. These places where the lip was thickened, as a temporary protection until the young animal decided to continue building, appear on the surface of adult shells, and persist as varices, "raised lines of growth," "longitudinal ribs," etc. Such forms of sculptural markings generally occur at equidistant points and preserve a scheme of regularity which would indicate that the periods of rest were of seasonal occurrence. All irregularities and all surface features of the external shell simply reflect certain peculiarities of the mantle margin of the animal; all spinous processes, for instance, as in the marvelous Murex tenuispina, indicate the existence of finger-like processes extending from the mantle margin, which once occupied the hollow thorns projecting from the shell. When the outer surface of a shell is devoid of any sculpturing and is perfectly smooth, one may assume that the mantle margin of the animal was simple. If the edge of the lip has a crenulated appearance and the surface of the whorls is decorated with revolving ribs, the mantle margin was probably folded or wavy. There can be no doubt but that the entire mantle surface is provided to some extent with glands for the secretion of shelly matter, for if any portion of the shell is accidentally injured the animal soon repairs the break with a calcareous deposit. But such repairs are never homogeneous in texture with the other parts of the shell of normal growth; the patches are never covered externally with an epidermis, and they are always devoid of color. The function of the epidermis is to protect the calcareous shell from the corroding agents contained in sea-water. Hence it is that, when the epidermis is removed, "dead shells" exposed to the influence of the water or the weather soon lose their brilliancy and luster, and become undesirable for specimens either for the cabinet or for study. It often happens that living shells, inhabiting a region where, for one cause or another, the water is highly charged with impurities of an acid nature, are discovered to be badly corroded about the apex, where the epidermis is thin and likely to be rubbed off. For this reason specimens collected in harbors near large cities or near the mouths of rivers are apt to be poor and defective.
The inner surface of all shells, if not actually nacreous, is [pg348] always perfectly smooth. When foreign substances, such as grains of sand, enter the shell and cannot be removed by the animal, the irritation caused thereby to the soft, fleshy creature induces discharges of a liquid from the glandular surface of the mantle, which hardens about the offending substance and glazes it over with a smooth, pearly deposit.
Acmæa testudinalis (page 343) presents a special type of gasteropod shell which is found in several families. Here the spire seems to be wholly absent, and the entire shell consists of but one large body-whorl. There are very many of these patelliform species, inhabiting many seas and belonging to many different genera, and in nearly all cases their embryonic shells display a spiral form. After birth the animal does not build his house upon the spiral plan, but expands the shell into one large shield-like covering. The student, however, must not presume that Acmæa is an ancestral type just because the simple character of the shell is suggestive of the model chosen to represent a schematic mollusk; anatomically Acmæa presents the complications of body-torsion which show a very considerable evolutionary change, and indicate that its simple shell is probably a degenerate form of a once more highly developed and convoluted one.
The forms, the architecture, and the painting of gasteropod shells are so infinite in variety that it would be unwise to attempt a description of their marvels. A close observer of nature's works soon becomes prepared for every surprise, but he never ceases to be charmed and fascinated by his new discoveries. The careful student alone can learn really to see and appreciate the wonders of nature, and this is especially true in the study of the Mollusca.
The Gasteropoda far exceed all the other divisions of the Mollusca in the number of their genera and species. Apparently this has not always been the case. There is evidence tending to show that in past geological epochs the pelecypods (the bivalve shells) outnumbered the gasteropods, but that in the course of time the increase in the genera of gasteropods has been more [pg349] rapid. To judge also from the very large number of extinct families among the cephalopods, which once flourished in astonishing abundance, it would appear that they are a dying race.
The classification of the Gasteropoda is primarily a division into three great orders: the Opisthobranchiata, the Prosobranchiata, and the Pulmonata. The Pulmonata, which include the land and fresh-water gasteropod shells, are characterized by the presence of a breathing-sac, or lung, instead of gills. They are, therefore, essentially air-breathers, and would perish if kept too long under water. The Opisthobranchiata and Prosobranchiata are marine, and, like all animals which breathe by means of gills, soon die when removed from the water. We have already noted, however, how some genera of marine gasteropods that find their station about high-tide mark are able to live for considerable periods out of the water, notwithstanding the fact that they are fully equipped with gills and not possessed of lungs. Indeed, there is one large family of prosobranchs (the Cyclostomatidæ) that has become entirely terrestrial in habit, its members having lost their gills and acquired lungs, but their organization otherwise is so essentially that of the prosobranchs that they have never been considered as pulmonates.
The main difference between these two orders of marine gasteropods is that in the prosobranchs the breathing-organs (the gills) are placed in a position forward of the heart, and the degree of torsion (page 330) characteristic of this molluscan class has been continued until the auricle of the heart is in front of the ventricle. There is always a shell, usually spiral, and, with few exceptions, an operculum. In the opisthobranchs, on the other hand, the relative position of the heart and gills is reversed, and they further differ from the prosobranchs in that the sexes are always united in each individual. The opisthobranchs are not always provided with a shell. One division of the order, known as the "nudibranchs," are entirely naked. These are commonly called the "sea-slugs," and are to be found crawling about the marine vegetation in shallow water, in tide-pools, and on the piling of old docks. Curiously enough, the sea-slugs (see page 354) have no gills at all, but, having lost these apparently essential organs, are [pg350] enabled to "breathe water" through their skin. They are wonderfully beautiful creatures, and the collector must not fail to secure some specimens, put them into a jar of sea-water, and watch them expand.
There are other more superficial differences between the prosobranchs and the opisthobranchs, which will enable even a beginner to distinguish them at a glance. When the latter are possessed of an external shell, it is bulbous, generally glassy, and with a simple lip, the aperture extending the entire length of the shell. Again, the mantle or the propodium of the foot is greatly extended and usually covers the shell almost wholly.
With the exception of the nudibranchs, which are common all along the Atlantic shore, especially north of Cape Cod, there are very few opisthobranchs to be found in American waters. Their shells are not very abundantly found anywhere, though in Florida there are two or three species which an untrained collector might discover.
Practically all the gasteropod or univalve shells that will be taken by the ordinarily expert collector along the shore are prosobranchs. This order includes the great majority of marine gasteropods, and is entitled to the distinction of claiming, in the great number of its genera and species, the most startling eccentricities of form and color to be found among the Mollusca, if not among all marine invertebrates.
Only those species which are actually abundant on the Atlantic and Pacific shores of the United States, and which are likely to be found by the untrained collector, are selected from the long list of forms which belong to the three faunal regions involved.
To avoid confusion, it is well to keep the systematic table in view:
| PLATE LXV. | |
|---|---|
| Bulla occidentalis, enlarged. | Bulla nebulosa. |
| Lottia gigantea, inside view. | Fissurella alternata. |
As has already been remarked, shell-bearing opisthobranchs are not abundantly represented in number of species in North American waters. South of Cape Cod, in muddy bays and well-sheltered places, the little shell, Haminea solitaria, may, however, often be found in considerable numbers.
Genus Haminea
H. solitaria. Like all external opisthobranch tests, the shell of this species is thin and fragile, and would appear to be of small importance to the creature it only partially protects. It is devoid of spire, is shining bluish-white, sometimes brownish in color, and is marked with revolving grooves, which, being cut across by irregular growth-lines, give its surface a faintly decussated appearance. The aperture is as long as the shell; the columella is incurved. This little species is especially abundant in shallow water about Vineyard Sound and in Peconic Bay.
Genus Bulla
B. occidentalis. This species, which is found in the waters of Florida on all sandy beaches, is a relative of the Northern species just described. The shell is larger than that of Haminea solitaria, and has a color-pattern of mottled or clouded reddish-brown on a white foundation. There is no spire; there is a pit in place of an apex; and the aperture extends the full length of the shell. To the naked eye the surface appears smooth and shining. It is an exceedingly variable species, occurring all through the West Indies, but not north of Florida. The shores of the keys along the west coast of Florida are often strewn with these shells after storms. The animal, like all the shell-bearing opisthobranchs, is very large as compared with the shell, and the large mantle folds recurve upward, almost completely hiding the shell from view. (Plate LXV.)
B. nebulosa. This is one of the largest and finest species of the family Bullidæ. It is found on the coast of California south of San Francisco. The shell is much larger than that of either of the Atlantic species mentioned above, but very closely resembles them in outline, although it is built upon a more generous plan and is more richly painted. It is brownish, mottled with white and yellow patches, and is very suggestive of certain kinds of birds' eggs. The animals have a greatly extended mantle which almost completely envelops the shell. The foot is extremely large, with great wing-like developments upon each side, called parapodia. Some species of Bulla have been seen to swim by means of the lazy flapping of the parapodia. (Plate LXV.)
SEA-HARES
Closely allied to these outer-shell-bearing opisthobranchs is a large and important group of tectibranchs, known as [pg352] "sea-hares." They have a mantle so greatly developed that it actually covers the shell, and its edges unite and fuse over the top. The shell, in consequence, having almost ceased to be of use as a protection, has degenerated into a mere horny plate, and has lost all resemblance to the ordinary gasteropod shell. Having practically lost its protective office and become a mere internal plate, it is quite probable that it will in time wholly disappear. The gills of the sea-hares are concealed under a flap of the mantle, their position being posterior to the heart. The most conspicuous representative of this type of tectibranchs in the United States is the following:
Genus Aplysia
A. Wilcoxii. This species appears at times in vast numbers in the waters of Florida, until the sea may truly be said to be fairly alive with them. They swim lazily with a waving motion of the parapodia. They disappear as mysteriously as they come, and for months not a specimen will be seen. There is a variety of this Floridian Aplysia which occurs at Cape May, but no sea-hares are to be found north of that point. The tropical Pacific furnishes an astonishing wealth of these creatures, belonging to many genera and species, and among them are some of the most beautiful of the invertebrate animals.
The opisthobranchs are divided into two suborders, the second of which is called Nudibranchiata. The name is well chosen and very suggestive, for it means "naked or exposed gills"; but this anatomical feature is only one of the peculiarities of this suborder. The nudibranchs are commonly known as "sea-slugs"; for, like the land-slugs, which are also true mollusks, they possess no shell at all. That they at one time carried a shell is evident from the fact that they are born with a rudimentary testaceous covering, which soon afterward disappears.
A striking peculiarity of the nudibranchs is that the conventional molluscan mantle is not usually apparent. Instead of seeing the usual flaps or folds of the mantle which more or less encircle mollusks, and which one seems to have a right to expect, [pg353] they are entirely absent, and the body of the sea-slug assumes in consequence a worm-like appearance. Other notable features of the nudibranchs are the great number of tentacular processes that usually project from the dorsal region of the animal, and, in many of the genera, an entire absence of gills. When the gills are present, as is the case in several of the nudibranch families, they are not placed along the side of the animal, where one would naturally look for them, but are arranged in the form of a rosette of plume-like processes situated in the posterior dorsal region, or, in other words, on the animal's back.
Nudibranchs are commonly to be found all along the Atlantic and Pacific coasts of North America, and more especially in the colder waters north of Cape Cod. They are essentially littoral, and live upon algæ in shallow water, upon eel-grass in sheltered places, and in tide-pools, where there is more or less vegetation. They crawl about the fronds of algæ, or swim, foot upward, with a gentle and undulating motion, or, when caught between tides, may be seen clinging to the under surface of rocks. Protective resemblances have been so remarkably developed in the nudibranchs that they are not always easy to discover; indeed, one may actually be looking at one for some time without suspecting it to be other than some torn fragment of seaweed.
There are many species belonging to several genera which frequent
the Maine and Massachusetts coasts, but those most likely
to be encountered are the following:
Genus Dendronotus
D. arborescens. This species is about one inch long and variable in color, but is usually reddish-brown or rose. There are no tentacles, but in their place are two antler-like appendages pointing forward and branched like a tree. All along the back are two rows of these curiously branched processes, which give to the animal the appearance of a plant. These cerata, or dorsal papillæ, are delicately transparent, contractile, and richly colored. The function of these papillæ is not fully known. As the animal has no specialized breathing-organs, it is reasonable to suppose that respiration is carried on through the outer skin and perhaps all over the surface of these branched papillæ. The liver, which [pg354] in most nudibranchs is extremely large and completely surrounds the stomach, in Dendronotus also extends into these dorsal cerata, so that they may have some sort of digestive function.
Genus Æolis
Æ. papillosa. This is probably the commonest nudibranch upon the North Atlantic coast, and it occurs as frequently in European waters. It may readily be found clinging to stones, algæ, the piling of wharves, eel-grass, etc., in bays or in rocky tide-pools. The color is yellowish-gray to orange, with purplish or olive spots. The cerata are very numerous, and cover the dorsal portion of the animal save for an open space along the middle of the back. There are two sets of tentacles, the two dorsal and the two simple labial tentacles. The foot tapers behind to a sharp point, and is truncate in front. As in Dendronotus, the liver is diffused into the cerata, which in Æolis are simple tubular processes without the branching character of those of the former. Both species lay their eggs in a gelatinous, bobbin-like cord, which is hung in festoons over rocks or upon zoöphytes, or at times they wind the cord in a coil upon the surfaces of stones. When the young first hatch out, they are provided with a glassy nautiloid shell, a fact which would indicate that at some remote period they were, like most mollusks, provided with a shell. Æolis swims in an inverted position, and is at times exceedingly active. It is very variable in coloring and in the number of cerata.
Genus Doris
D. bilamellata. In Doris we find a quite different type of nudibranch from that presented by the two genera Dendronotus and Æolis. In one there were numerous branched cerata or dorsal papillæ, in the other many plain cerata, and in neither case were there any specialized [pg355] breathing-organs. In Doris, however, there are no dorsal papillæ at all, the back being covered with calcareous spicules, which form a sort of secondary shell. There is a circlet, or rosette, of retractile plume-like gills, or branchiæ, placed in the middle of the back posteriorly. There are various species of Doris, and of closely allied genera belonging to the family Dorididæ, which are to be found along the New England coast in tide-pools and among rocks. Those seen by the writer never appeared to be other than very sluggish creatures, and proved to be rather unsatisfactory tenants of the aquarium.
When discovered at low tide upon moist seaweed or stones, or when disturbed in the water, nudibranchs often appear like small lumps of jelly-like tissue, without a single attractive feature. It is only when they are placed in a jar of sea-water and left unmolested for a few moments that they unfold their beauties to the view. On the Pacific coast there are numerous species of nudibranchs, occupying corresponding shore stations and exhibiting the same degree of high coloration. There are over a thousand species of sea-slugs scattered about the various seas. They find their greatest development in tropical waters, and for brilliancy of tint and variation in form are unequaled by any other class of animal life, save perhaps the butterflies.
To this exceedingly large and comprehensive order of gasteropods belong the great majority of univalve mollusks. In point of number of genera and species and in abundance of individuals, the prosobranchs probably exceed all the other orders of mollusks combined. They may be called the typical gasteropods, the shell being univalvular and generally spiral, and the animal exhibiting the singular torsion of the visceral mass which has placed the breathing-organs forward of the heart. They are provided with an operculum (placed upon the posterior dorsal portion of the foot), which is used to close the aperture of the shell against [pg356] enemies when the animal retires within its fortress. They are all marine except a very few families, which, supposedly of marine derivation, have become terrestrial in habit.
The prosobranchs are further subdivided into suborders according to certain peculiarities of the heart and breathing-organs. There is a group of these prosobranchs which gives evidence of an inferior degree of that visceral torsion which is always found in the gasteropods. In this group, or suborder, the heart has two auricles, and there is a pair of gills instead of only a single one. Other internal organs are paired just as they were represented to be in the schematic mollusk. This group also seems to show its primitive character in the want of a proboscis and a siphon, or, in some families, by having the ventricle of the heart traversed by the intestinal canal, just as in the lower class of mollusks, which includes the clams and oysters. For the most part the shells of this group are not typically spiral, but are patelliform, shield-like coverings, with only a suggestion of a spiral form at the very tip of the apex. This group of primitive prosobranchs is included in the following suborder:
This suborder is named from the presence of two auricles in the heart.
Family ACMÆIDÆ
The first family to be noted is the Acmæidæ. Its principal genus, Acmæa, is well represented on both the east and the west coast of the United States.
Genus Acmæa