O. mutica. In Floridian waters this species is exceedingly common. It is not more than one fifth of an inch long, is highly polished and shining, and yellowish-white in color, with revolving pale rufous bands. It has the typical oliva-shape.
| PLATE LXXVII. | |
|---|---|
| 1, Marginella apicina, enlarged. | 3, Olivella biplicata, enlarged. |
| 2, Oliva literata. | 4, Conus floridanus. |
| 5, Cancellaria reticulata. | |
O. biplicata. A Californian species of considerable interest, the largest of the olivellas. It was evidently used by the aborigines as wampum or as ornaments, for a great many specimens are found among buried Indian relics. This species is about the size and shape of an olive. The spire is short, though longer than the typical Oliva spire, the shell is smooth and highly polished, and the columella is thickly calloused, and has two entering folds near the base. The color is creamy-white to dove-color, with a purple sheen over all, and purple markings [pg401] about the base. Professor Keep speaks of the vast numbers of these olivellas on sandy stations along the Pacific coast. Having found a place where a "little stream of water was oozing out from the bank of sand," he proceeded to dig with a hoe. "I found them [O. biplicata] by the hundreds," he continues, "and I had gathered about a thousand before the tide came in. They seemed to lie in groups just under the surface of the sand, yet wholly concealed from sight." (Plate LXXVII.)
O. boetica. A much smaller Pacific form, with a higher tapering spire. It is brown to bluish in color, and often has yellowish stripes. It is polished and shining. Found in sandy stations.
Family CONIDÆ
Genus Conus
This is a comprehensive family of several highly diversified genera. Its most prominent genus is Conus, a name which indicates the principal feature of the shell, for it is almost an exact cone in shape. Conus, along with Voluta and Cypræa, is entitled to the honor of being considered an aristocrat among mollusks. There are about three hundred and fifty species known, mostly tropical and Indo-Pacific in distribution. The shells have many points of beauty and are often of very high coloration and eccentric markings. Some of the rarer forms are famous in conchological annals for the enormous prices which they have commanded. The rare and beautiful Conus gloria-maris once brought £43 ($215) at an auction sale in London. In all shells of Conus there is a notch at the upper edge of the aperture for the accommodation of a posterior canal. In some other genera of this family this notch becomes a more prominent feature. The animal has a well-developed foot, a retractile proboscis, eyes situated upon the tentacles, and a fairly long siphon. Upon the under surface of the foot is a conspicuous pore, which opens into a water-vascular system of the foot. Conus is accused of having a poison-gland connected with the radula and of having shown vicious traits when captured. The animals are shy, and remain most of the time in hiding, while their pretty shells during life are generally rendered obscure by a dull, colorless epidermis. There are but few species of Conus upon the Atlantic shores of the United States, and these are confined to the warmer waters of Florida and the Gulf of Mexico. [pg402]
C. floridanus. A not unusual shell in shallow-water stations throughout the Florida coast. It has almost the exact outlines of a cone. The flattened shoulder is marked off by an acute carina; the spire is low, but many-whorled and pagoda-like; the aperture is long and narrow, and the lip is simple. The coloring is yellowish, banded on a creamy-white background, or dotted in circular rows. Length two inches. (Plate LXXVII.)
C. pealii. One of the smallest species of this genus, its length never exceeding one half of an inch, whereas the majority of the cones are over two inches in length, and many of them rank as large shells. This little Floridian species is regularly cone-shaped, with carinated whorls, thin outer lip, and pagoda-like, low spire. The color is reddish-brown with sky-blue spots, or bluish-brown with lighter spots. There are encircling grooves about the base of the shell.
Family CANCELLARIIDÆ
Genus Cancellaria
The last family of the Gasteropoda to be considered is the Cancellariidæ, which has but one genus, Cancellaria. There are several species upon the east coast belonging to deeper-water zones, but one well-known species, C. reticulata, is very common on shore stations from Hatteras south. The shell is oblong and solid, with a moderate spire, and whorls slightly flattened below the sutures, and is very roughly granulated or reticulated. The aperture is narrowly ovate, and is ribbed inside. The columella has two very strong and prominent plaits. The color is white, banded and variegated with reddish-brown. Length an inch or more. (Plate LXXVII.)
In nearly all collections of shells from the northeast coast of the United States are certain specimens about one to one and a half inches long, pure white, and shaped very much like an elephant's tusk. They are round, hollow tubes, slightly curved, of larger diameter at one end than at the other, and with an opening at either end. There are two very common species of these tooth shells upon the New England coast, [pg403] Dentalium dentale and Entalis striolata. The first has a simple round hole at the smaller end of the shell, and is faintly marked by longitudinal striæ; the other is a smooth shell which has a notch-like fissure on the margin of the apical perforation. Neither of these species is, strictly speaking, a littoral form, for all the Dentalidæ range into deep water, many of them living only in the profounder depths of the ocean. But these two species are exceedingly common in the New England coastal waters, at very moderate depths, and may sometimes be found upon the beach cast up by storms. They live buried in the mud, and feed upon infusorians and all manner of microscopic organisms.
Upon the west coast Dentalium pretiosum is very abundant north of California. It is almost like the east-coast Dentalium, but is more slender. The Indians used to gather these shells and string them together upon long threads to be carried about and used as money. In California occurs Dentalium hexagonum, a very delicate little species with a slightly angulated shell.
The animal of the Dentalidæ is remarkable, and easily merits the rank of a separate molluscan class. It has no head, no tentacles, no eyes, no heart, and no gills. It is a mollusk because it has a mantle, a foot, and a radula. Its position, therefore, is between the Gasteropoda, which it resembles in its univalve shell and radula, and the Pelecypoda, to which it is related by the pointed foot and the absence of head and tentacles, and also by the symmetry which pervades its organization.
Upon either side of the mouth, just beneath the flap of the mantle, are bunches of ciliated, contractile filaments (captacula), flattened at the end, which are supposed to be breathing-organs, and are perhaps exserted for the purpose of catching food. [pg404]
The entire absence of such important organs as the heart and gills, together with the fact that they are undoubted mollusks, would indicate that the Scaphopoda are very degenerate animals. The circulatory and respiratory system of mollusks is typically of a high order.
A great many specimens of the Dentalidæ dredged in New England waters are dead shells, occupied by a worm which seals up the larger opening, leaving only a small aperture through which it protrudes its long white body. These must not mislead the collector into a vain search for a mantle and other molluscan characteristics of anatomy.
Some of the larger species of Dentalium from deep-sea stations are four to five inches in length. [pg405]
This is a large and important class of the Mollusca, comprising an extensive group of animals, which vary widely in structure. Next to the Gasteropoda, it is the largest class of the phylum in number of genera and species. The name indicates a "hatchet-shaped" foot, but in many cases it is a misnomer. Acephala, meaning "without a head," is also employed for this class, and is, perhaps, a better term. Lamellibranchiata, referring to the type of gills prevalent throughout the division, is another name often used. The popular name "bivalves" is an altogether correct one, for all the animals of this class have two shells. To the Pelecypoda belong the oysters, clams, scallops, mussels, and, in short, all the bivalve mollusks (the word "valve" meaning "shell"). The organization of a pelecypod is entirely similar to that of a gasteropod or a chiton in its fundamental or essential plan, but it differs widely from both in matters of detail. If the student will keep in mind the general principles of gasteropod or the amphineuran structure when he dissects his first pelecypod, he will quickly see that the latter is about the same as a chiton would be were it folded over from a longitudinal median line as axis along its back, so that the opposite mantle-edges would meet, and its various shell-plates would unite upon either side into a single valve; or it is very like a gasteropod made symmetrical, and covered upon its two sides by separate shells instead of covered over its top by one shell. The loss of head, eyes, and tentacles, the substitution of labial palps, the extension and greater development of the gills, the modification of the foot and mantle, and the presence of certain special glands in the Pelecypoda become mere matters of anatomical detail. [pg410]
Any large clam will serve for dissection. In the neighborhood of New York the edible species known as the "Little Neck" clam (Venus mercenaria), or the larger Mactra of the sandy shores, will answer the purpose very well. In Florida, Callista, Venus, and Cardium are all available. On the west coast the large, heavy Tivela crassatelloides is probably the best subject. The oyster (Ostrea) is a degenerate type; and the scallops (Pecten) and the mussels (Mytilus and Modiola) have certain special modifications of their organs which might prove confusing, so these genera are not recommended for dissection and study at first.
Boiling is apt to shrivel and distort the soft, fleshy animal of the bivalves, and it is far better to examine a fresh specimen. The surest way of opening a clam without injuring the animal is to break one of the shells by sharp taps of a hammer, using great care not to lacerate the body within by a too vigorous assault. Pick off the broken pieces after having separated them carefully with a knife from the mantle margin, to which they cling, and after having cut through the tough adductor muscles as close to the shell as possible. The subject for anatomical study is then prepared "upon the half-shell."
The mantle is generally very thin, often a fleshy film of the finest tissue, and adheres to the inner side of each valve. The outer rim of the mantle is thickened and free, i.e., is not attached to the shell. This free portion is capable of slight extension beyond the margin of the shell when the valves are opened and the animal is off guard. In many genera the mantle edge is highly ornate, being waved, crenulated, or fluted, or is beset with several rows of papillæ, and is often richly colored. Mr. Hickson, a naturalist, who traveled in Celebes, says that the brilliant coloring of the mantle margins of mollusks contributed largely to the extraordinary color-effects upon the coral reefs.
It will be seen that the mantle entirely incloses the animal at the back and sides, just as the cover of a book incloses the pages or printed portion. At the posterior end of the animal there is usually a point—or perhaps two points—at which the flaps of [pg411] the mantle lobes unite in order to form two openings. These openings are called the siphons.
In some cases the mantle lobes do not actually unite to form regular siphonal openings, but in life the free mantle-edges have a way of adjusting themselves posteriorly to form functional siphons without actually coalescing. Usually, however, the mantle flaps not only unite posteriorly to form true siphons, but are capable at that point of varying degrees of protrusion from the shell, and when extended the siphons appear as two tubes. In some genera these siphonal tubes are very long; in others they are fastened together and surrounded by a tough, leathery integument, [pg412] which, like the siphons proper, is only an extended portion of the mantle-edge. The orifices of the siphons are generally papillaceous. These two siphons (for there are always two if there are any at all) are the anal or excurrent (upper) and the branchial or incurrent siphon. The function of the latter is to draw in the pure water to bathe the gills and to furnish food, while the office of the former is to eject waste materials and the water which has already passed over the gills.
Besides the points of juncture where the mantle-edges unite to form the siphons, the coalescence of the two mantle lobes may be extended, and they may become further united and fused together at other points. Indeed, the fusion of the mantle-edges may become almost complete; but it always leaves the siphonal openings and a third opening through which the foot may be extended. This opening for the foot is called the pedal opening, or orifice. In such cases, therefore, the mantle becomes much like a bag or sac. The degree of fusion of the mantle-edges appears to correspond closely with certain changes in the organization of the animal, and this feature must be regarded as a very important character. [pg413]
The figures on pages 411, 412, and 413 show the degrees of mantle fusion in various groups from the open to the almost wholly closed types. One (A, page 413) gives an example of very large siphons united together and protected by the tough, leathery integument. To the right of this cut may be seen the foot projecting from the pedal opening in the mantle. Another gives the two siphons of Cardium, the lower or branchial one being the longer. The mantle-edges below are fluted, and a large foot projects forward. Another shows two siphons of Scrobicularia, of the free and greatly extended type. Fig. B (page 413) illustrates the ventral portion of a pelecypod, which shows very well the most advanced type of mantle fusion. Besides the siphonal openings at the end of the long projection to the left, the only other opening is the very small orifice marked "2," for the accommodation of a small, insignificant foot. In the other figures different degrees of mantle fusion are indicated.
Before removing the mantle from one side of the specimen to be examined, note the two strong muscles which are at either end of the body. These are the anterior and posterior adductor muscles, their office being to hold the valves tightly shut. They are of great strength, as any one may prove to his satisfaction by attempting to open a clam. Through a long series of forms these two muscles approach each other in position, and in the scallops and the oysters there is but one adductor muscle, occupying a central position. Morphologically it is the survival of the posterior adductor.
Removing carefully the thin, fleshy mantle, the gills are exposed to view—that is, of course, the gills of that side of the [pg414] animal, for there are two sets, one upon each side of the body, or visceral mass, and lying between the body proper and the mantle folds. Each set consists of two plate-like bodies with a texture of reticulated or basketwork appearance. Thus there are the inner and outer right gills and the inner and outer left gills. If one gill is removed and carefully examined it will itself probably be found to be double, consisting of many filaments placed side by side and then doubled back like a row of hairpins, the filaments being united by interciliary processes, or by vascular channels together with more or less dense connective tissue.
The modifications of the pelecypod gills are difficult to follow, but the principle upon which they perform their duties is the same in all cases. The gill-filaments are all connected with a long vein, and, being hollow, admit the blood, which is aërated by close contact with the water and is then returned to another vein in immediate connection with the auricles of the heart. There is another function accomplished by the gills, which in some families seems to be quite as important as their respiratory one, namely, the office of giving lodgment to the ova while in process of development before hatching. At certain seasons the gills of a number of pelecypod genera become literally filled with eggs; sometimes this curious phenomenon extends to the mantle itself, and more or less to the entire surface of the animal. The eggs are first regularly ejected from the genital ducts and find lodgment upon the body-surface, but usually only upon the gills, where they remain as in a brood-pouch between the lamellæ of the inner and outer gills. When the eggs hatch, the free-swimming young escape from the mantle cavity. Probably not more than one individual in a million ever reaches maturity.
The figure on page 408 shows the mantle removed, exhibiting the gills, the foot, the labial palps, and the pericardium, inside of which is the heart. Removing the gills, we find exposed the body, or visceral mass, which, as in the Gasteropoda, is thickened below into the foot. A longitudinal section is shown on page 408, the visceral mass being sliced almost through the middle; a portion of the gills of the farther side shows below. [pg415]
The foot undergoes great variation, from a strong, powerful organ capable of forcing its way through heavy gravelly bottom or burrowing deeply into the sand or mud, to the merest trace of a pedal organ. In the oyster the foot has become almost entirely atrophied; the stationary life led by the animal renders such an organ quite unnecessary. Excepting in a few families, the pelecypod foot is well developed. In a number of genera the foot contains a gland for the secretion of long silk-like or horn-like fibers, which are collectively called a byssus. The use of a byssus is for attachment to any object to effect a temporary or permanent lodgment. The accompanying cut shows Mytilus edulis, a common east-coast pelecypod, attached by its byssus to a piece of wood. Most bivalves having a strong byssus exhibit a feeble development of the foot; nearly all bivalves, however, show traces, sometimes only in the embryo, of a byssal gland.
The visceral mass contains the liver, the exceedingly large generative glands, the kidneys, etc., and through all this soft whitish or reddish mass the alimentary canal wanders about in a tortuous fashion, finally passing through the pericardium and the ventricle of the heart, and terminating just over and back of the posterior adductor muscle. [pg416]
The pelecypods have no head or tentacles, but the place of the latter is taken by two pairs of triangular flaps upon either side of the mouth. These are called the labial palps, and they no doubt operate by means of their ciliated surface to keep in motion the current of water over the gills and to the mouth.
As a rule, pelecypods have no eyes, but when eyes are present they are situated upon the edge of the mantle. The eyes are not highly developed organs, but they nevertheless appear to be very sensitive to light, for the bivalves which are so endowed will, when kept in aquaria, instantly close their shells when the shadow of a fish passes over them.
The heart lies in the pericardium and is situated dorsally, just in front of the posterior adductor muscle. It has a median ventricle and two lateral auricles, each connected with the branchiæ upon its respective side. It seems a curious economy that passes the intestine through the ventricle, but this is the case in the great majority of pelecypod genera. The vascular system is closed and is of a high order. It is probable that "breathing" takes place upon the inner mantle surfaces as well as in the gills themselves; for these surfaces are crowded with excessively fine and delicate capillaries, which bring the blood very near to the water. This power of "breathing by the skin" is quite characteristic of the Mollusca in general.
Having once familiarized one's self with the more prominent features of pelecypod organization, it becomes very interesting to examine anatomically every new form that is captured. Surprises are always in store, and sometimes the peculiar juxtaposition and relative sizes of the organs are a trifle confusing; but, upon the whole, the bivalve organization is simple and lends itself far more readily than does that of the gasteropods to satisfactory examination. Some of the more striking anatomical peculiarities are reserved for mention in the notes upon the various genera and species common upon our shores.
As has been seen, the testaceous covering of the Pelecypoda consists of two valves or shells which fit more or less closely and [pg417] firmly together and are placed one upon either side of the animal. The two valves are always held together tightly along a dorsal margin by a "hinge ligament," an exceedingly tough, leathery substance, and they are usually secured the more firmly by a system of interlocking teeth, which project from the hinge. The opening and shutting of the valves is controlled by the adductor muscles of the animal, by the ligament which binds the valves together, and by the cartilage, an elastic pad of rubber-like appearance which is lodged just within the hinge, and is compressed when the valves are closed together. Thus, when the valves are closed, there is always a strain upon the adductor to overcome the elastic resistance of the cartilage. When a pelecypod dies and the pull of the adductor muscles is released, the valves gape open.
The summit or apex of the valve is called the umbo, or beak. It is usually twisted to some extent, and in certain forms develops a suggestive spiral figure. The beaks of the valves are the points where the shell-growth begins, the secretion of calcareous matter being made by the mantle margin and deposited about the edges of the valves; the growth is therefore constantly away from the umbones.
The umbones usually point forward. In many species they touch each other over the hinge-margin or approximate very closely; in other species the hinge-margin is very wide and the beaks are comparatively far apart. The hinge-margin, lying adjacent to the umbones, is known as the dorsal margin of the shell, in contradistinction to the ventral margin, opposite the umbones. The anterior margin is the front edge and the posterior margin the hinder edge of the shells, through which the siphons may project.
When the umbones are about central in respect to the posterior [pg418] and anterior margins, the shells are said to be equilateral. When the two valves are almost exactly alike in size and shape they are called equivalve.
In many forms there is a heart-shaped space upon the dorsal margin of the closed valves forward of the umbones, called the lunule. Similarly placed back of the umbones is sometimes a more or less clearly defined space called the escutcheon. The outer surface of bivalve shells admits of infinite variety of sculpturing and ornamentation, ranging from a perfectly smooth to a heavily ribbed, nodose, spinous, or deeply decussated surface. The color-markings are often brilliant and eccentric.
The right and left valves may be distinguished by remembering that the siphons are always posterior, and that the umbones usually point forward.
The hinge of bivalve shells undergoes many variations in the different genera. The simplest type is a smooth edentate surface where the two valves meet and are held together only by the strong ligament. Generally, however, there is a system of interlocking teeth, those in the center of the hinge being the cardinals and those upon either side the laterals. In some forms there are rows of fine comb-like teeth along the hinge-margin, with no distinction between cardinals and laterals.
The object of these hinge-teeth is at once obvious, for they give a rigidity and increased strength to the entire mechanism, especially when closed. There is occasionally a saucer-shaped plate or platform just under the cardinal teeth and within the shell, called the fossette. Upon this rests the internal elastic cartilage.
Upon the smooth interior surfaces of the valves are always certain markings, which correspond to anatomical features of the animal and are consequently of considerable importance in the determination of the systematic position of the specimen. First, the well-marked impressions left by the adductor muscles will be noted; then a more or less distinct line which, describing roughly a circle, connects the two muscle scars. This line represents the points at which the mantle became detached from the shell. In other words, all the space within this pallial line (barring the [pg419] muscle scars) was, in life, covered by the mantle, which adhered tightly to it. The space between this pallial line and the outer edge of the valve was occupied in life by the free portion of the mantle. Frequently a depression in the pallial line toward the center of the valve is seen in the posterior end. This is called the pallial sinus, and marks the space occupied by the siphons. The pallial sinus is absent, moderate, or deep, according as the animal possessed no siphons, or small or large ones.
The length of bivalve shells is measured from their anterior to their posterior ends, while the height indicates the greatest diameter between the umbones and the ventral margin.
The greatest possible degree of variation exists in the structure of the pelecypod shell. Every degree of thickness from the most fragile, tissue-like structure to the very heavy and ponderous tests of some of the Veneridæ is to be found. There is also a range in size from the very minute Spheria to the giant Tridacna of Eastern seas, which weighs several hundred pounds. As in the Gasteropoda, there is fortunately a very close parallelism between shell and anatomical variations. One very quickly learns to place a bivalve in its systematic position by a mere glance at the shell alone. While the higher classification into orders, suborders, etc., is based upon anatomical features, the genera often and the species always are founded upon conchological or shell characters.
The Pelecypoda offer one of the most interesting fields for investigation and study among the invertebrates. Although by no means neglected, they have not received as much attention on the part of biologists as has been given to other phyla, or indeed to the other classes of this same phylum.
Nearly every systematic writer upon the mollusks has attempted to give a good classification of the bivalves, yet none of the results is wholly satisfactory. The scheme now generally adopted is one based upon modifications of the gills, or branchiæ, and has practically replaced all the older classifications, which depended [pg420] upon the presence or absence of siphons, the degree of mantle fusion, the arrangement of teeth upon the hinge, the number of adductor muscles, etc. The idea of arrangement according to gill-structure is substantially this: the development of the gill from the simplest and most rudimentary type through successive stages to a higher, more complex, and presumably more efficient type, marks the natural progress or development of the pelecypod animal itself. By adopting the gill as a guide one follows, therefore, a natural method. Upon the other hand, the presence or absence of siphons, the shape of the foot, the number of adductor muscles, all depend merely upon the acquired habits of the animal, these particular features being subject to modification according to environment and changed conditions.
The five orders of the Pelecypoda are: Protobranchiata, Filibranchiata, Pseudolamellibranchiata, Eulamellibranchiata, Septibranchiata.