| Ideal or schematic mollusk. |
|---|
| 1, dorsal view: h, head; t, cephalic tentacle; sh., shell covering visceral mass; b.m., outline of body-mass; r.m.e., edge of mantle; c.t., ctenidia, or gills; q.a., generative apertures; r.e., renal openings; e.o., excretory opening; f, posterior portion of foot; o.e.m., edge of shell. |
| 2, ventral view: h, head; t, tentacle; m, mouth; b, outline of body; e.m., edge of mantle; s.f., lower surface of foot; c.t., ctenidia, or gills; f, posterior portion of foot. |
| 3, horizontal section, showing heart, nephridia, and gonads: m, mouth; o, esophagus; g, gonad; p, pericardium; n, nephridium, or renal organ; v, ventricle; a, auricles; g´, gonad aperture; n´, renal opening; e, excretory opening; f, posterior portion of foot. |
| 4, vertical median section: m, mouth; b.m., buccal mass; l, liver; d.a., digestive tract; q, gonad; f, foot; q.o., gonad aperture, or opening of oviduct; r.o., renal opening; e.o., end of digestive tract, excretory opening; n, mantle; r, nephridium, or renal organ; o.f., recurved portion of mantle; sh., shell; v, ventricle; p, pericardium.] |
The digestive tract, beginning at the mouth, opens immediately into a muscular throat, which is called the buccal mass. Within the buccal mass there is a long, fleshy ribbon that bears upon its upper surface numerous rows of sharp teeth; these rows of teeth run crossways on the ribbon, each row containing more or less prominent median teeth, with several laterals upon each side, and with marginal teeth on each side of the laterals. This is called the radula, and is used for tearing or rasping the food. When [pg319] not in use it lies coiled up within the buccal mass. Beyond the buccal mass, food passes into the esophagus, which widens out into a muscular pouch or stomach; this may also be provided with chitinous teeth or hard knobs or plates for the purpose of further triturating the food. Above and partially surrounding the stomach is a large reddish or brownish liver, which furnishes to the alimentary canal the digestive juices. From the stomach the intestine leads to the terminal anal opening. There are two organs, serving the function of kidneys, placed dorsally one upon each side of the body (nephridia). A heart lies within the median line of the body, and consists of a ventricle and two lateral auricles. The vascular system is almost entirely closed; only in so far as it is partially open does it essentially differ from that of higher groups; the blood is whitish or light bluish, and receives its oxygenation in the gills or along the free mantle surface, through which it courses in exceedingly thin capillaries. In this manner respiration is carried on to some extent throughout the whole outer surface of the body. There are two branchiæ (gills). These have the appearance of small feathers and are placed upon each side of the animal posteriorly; they project slightly. Spread over the top of this animal, fitting it perfectly, is a thin fleshy covering attached to its dorsal portion like a skin, but hanging free, like a flap, over the sides, the head, and the posterior end of the animal. Upon the sides it may just cover the tips of the branchiæ, but in front the head is seen to project from under it. The loose-hanging portion of this sac-like covering is somewhat thickened, and perhaps recurved upward upon itself. Upon the edge of this flap all the way around may occur a row of cilia, or tentacular processes. This cloak-like covering is called the mantle, and, as already noted, is the most peculiar and persistent characteristic of the Mollusca. The "cavity" formed between the loose-hanging portion of the mantle and the sides of the foot is called the mantle cavity, and contains posteriorly the gills, or branchiæ. From the outer surface of this mantle a calcareous secretion takes place, forming a shell, which, in our ideal mollusk, will be shaped in conformity with the surface outlines of the mantle, oblong and convex above. Over the lower portion of the shell the edge of [pg320] the mantle recurves. When the creature is disturbed or is at complete rest it may withdraw that portion of the mantle which protrudes from below the shell entirely within the edge of the shell, so that the "soft parts" may be wholly covered, and thus protected from harm. In such a state of rest nothing may be seen from above but a shell. The entire upper surface of the mantle is firmly attached to the under or inner side of the shell, except the mantle margin, this being free.
The nervous system consists of a circumesophageal ring of nerve-cells, with two cords passing along the digestive tract to the posterior end of the animal. In the head and above the buccal mass are several series of nerve-ganglia. These also communicate with the posterior portion of the animal through two parallel cords which pass along its ventral side. From these four principal branches, which traverse the entire length of the creature, minor nerve-cords are thrown off to the tentacles, the foot, the branchiæ, and the various organs of the viscera. The sexes are separate.
Reviewing our "typical mollusk," we find that it is bilaterally symmetrical—that is, uniform upon both sides of a median line from mouth to excretory opening; is unsegmented; has the ventral surface thickened into a creeping foot or disk; and is possessed of a head with tentacular processes, a radula, lateral branchiæ, a heart with a ventricle and two auricles, a well-developed mantle, and a shell.
If the reader can learn these few details of organization he knows the structure of a mollusk. In turning to the actual living forms he will find merely modifications (though sometimes confusing ones) of these various parts.
The Mollusca are divided into five great classes: Amphineura, Gasteropoda, Scaphopoda, Pelecypoda, and Cephalopoda. These names, it will be noticed, with the exception of the first, all refer to the foot, the modifications of the foot being selected as the basis of the first great divisions of the phylum. The Gasteropoda crawl upon the thickened ventral surface of their bodies (already [pg321] referred to as the foot), as the common garden-snail, the periwinkles, and in general all those mollusks which have a spirally coiled shell. The Scaphopoda have a long, worm-like foot, with which they burrow in the sand or mud. Their shells are like miniature elephant-tusks, but are open at both ends. The Pelecypoda have a more or less club-shaped foot, utilized, in the many families, for a great variety of purposes. They are always inclosed in a bivalve shell. Familiar examples are the oysters, the clams, mussels, etc. The Cephalopoda have the foot modified into a number of arms, which encircle the head or the mouth. They are the cuttlefishes, the octopi, squids, etc.
The Amphineura, as already observed, approach most closely to the ideal mollusk just described. They are bilaterally symmetrical. This fact is so important that it constitutes them a class, notwithstanding the fact that in respect to the foot (the basis of division into classes) they would be included with the Gasteropoda, for (barring some exceptional instances) they creep along upon a foot quite as our ideal mollusk would, and as the Gasteropoda do. The head carries no tentacles, thus essentially differing in this respect from the ideal form. The mantle is extended down in front, completely covering the head. The branchiæ are confined to a few pairs of ctenidia, or plume-like gills, within the mantle groove or cavity, and are arranged upon each side of the excretory opening like small feathers.
There are two orders of the Amphineura, the Polyplacophora and the Aplacophora (or Solenogastres), the one name meaning "bearing many plates," and the other "without plates," the word "plate" in this sense being synonymous with "shell." The shell of the first order consists of eight calcareous disks arranged in a longitudinal row along the back or dorsal side of the animal, which overlap like shingles on a roof and admit of great variation of form in the various families. [pg322]
It will be observed from the examples given that the shell plates do not entirely cover the mantle, but that a considerable portion of the upper mantle surface is left exposed along the side of the animal. This exposed portion of the mantle is called the girdle. It is always very much thickened and is often exceedingly tough. In some species it is covered by a great number of calcareous spicules or granules which are embedded in the leathery mantle tissue. Strangely enough, the Chitonidæ (a large family of the Polyplacophora) possess a great number of eyes scattered over the shell plates like minute dots.
The members of this order are to be found in all parts of the world, but their greatest development is upon the west coast of America. In California the student of marine life has a splendid opportunity to examine the chitons. They are generally to be found adhering to the under surface of stones about low-tide mark. They are very sluggish in their movements and may readily be removed from their resting-places by suddenly thrusting a knife-blade under the foot and prying them off. When thus detached they have a habit of rolling up into a ball and so remaining indefinitely; to prevent this they should be placed at once in a jar of sea-water, where they will resume their natural position. For preservation as cabinet specimens, chitons should be tied down between two boards or flat substances, to hold them firmly extended until dried; otherwise they are apt to contract out of shape.
Six or eight species are exceedingly common upon the west [pg323] coast. The collector who has successfully taken a number of these will not fail to note the varying degrees of width of the girdle in the different species. In some cases the girdle partially covers the overlapping shell plates. The largest of all the chitons inhabits the California shores and is known as Cryptochiton stelleri. It measures fully six inches in length and three in width. In Cryptochiton the girdle is continued over the shell plates as a tough, gritty, brownish covering. The shell plates are white, and, as detached pieces, are frequently to be found cast up upon the California beaches, where, from their curious shape, they are often referred to as "butterfly-shells."
On the east coast of America there are comparatively few species, and these are all small and inconspicuous. The three commonest ones are Chiton ruber, C. albus, and C. apiculatus. They may readily be found on stones or dead shells fished up from below low-tide mark, or in rock-pools left by the receding tide.
This second order of the Amphineura is entirely destitute of the shell plates characteristic of the first. The outer mantle surface, however, develops a number of calcareous granules which correspond to a shell. The Aplacophora are symmetrical, having the mouth and the excretory opening at the anterior and posterior ends respectively. The foot is greatly modified, being narrowed at times to a mere slit along the ventral surface. Some of them have long, worm-like bodies. This is no doubt a very primitive form of mollusk, or, if not a primitive form, it has greatly degenerated from some higher type. They are supposed to be commensal in habit and live in the deeper water offshore.
The Aplacophora are not littoral animals, and it is extremely unlikely that one will ever encounter them upon the beach, unless, possibly, after the severest of storms. We may therefore leave them with this mere mention and proceed to the second class of mollusks, the Gasteropoda. [pg324]
The class Gasteropoda is the largest and most comprehensive subdivision of the Mollusca, and within its wide range of families many differing details of organization are to be found. As a class it possesses all the most characteristic features of the phylum, though it is frequently modified in a high degree. This class includes all the univalve mollusks (except Nautilus and Dentalia), such as the snails, the whelks, and the host of spirally coiled land, fresh-water, and marine shells.
The gasteropod foot is, as has been remarked, primarily the same as in our schematic mollusk—a flat, muscular disk caused by the thickening of the ventral body-surface. The neck, head, and tentacles are also quite the same, but considerable modification of these organs will be found when we come to examine some specimens. The mantle is always present, except in the nudibranch or non-shell-bearing forms; but in few of the Gasteropoda is the mantle so regularly simple as in our ideal mollusk.
Perhaps the most striking feature of the Gasteropoda, and one that will at first surprise him who has in mind the simple structure of the ideal mollusk, is the fact that they are always asymmetrical—that is to say, a median line drawn longitudinally through a gasteropod will not divide it into halves of similar anatomical structure.
The quality of symmetry is an important one throughout the lower orders of animal life. In nearly all phyletic or class descriptions the word "symmetry" occurs, and its exact meaning must be understood. Take, for example, a human being; a median line drawn vertically would divide him into two similar halves—upon each side would be an eye, an arm, a leg, etc., of similar shape and construction. So far at least as the external features [pg329] go, man is then a symmetrical creature; likewise the vertebrates in general. Cases of actual symmetry are found in the lobsters and crabs and in the segmented worms. It is usual in describing mollusks broadly and in general terms to call them symmetrical animals, yet here is the largest class within the phylum, whose representatives are none of them symmetrical. In the Gasteropoda the mouth is anteriorly placed in the head, just as in the ideal form, but the digestive tract, after traversing the visceral cavity in the usual fashion, suddenly turns forward and terminates in an excretory opening either on the right or on the left side of the animal, just back of the head. The various other orifices for the genital and renal glands are also placed in this unexpected anterior position. The gills, or branchiæ, are to be found upon one side only, forward of the heart, and differ somewhat in appearance from the plume-like processes of the Amphineura.
What, then, has become of the corresponding gill we should expect to find on the other side? The symmetrical shape of the foot is not in the least altered, but what has happened to cause this distortion of the visceral portion of the animal? And, finally, will this torsion of the body of the snail perhaps account for the spiral nature of the shell?
An evolutionary process is supposed to have taken place in mollusks, which, if real, would no doubt account for these curious conditions of torsion in the Gasteropoda; but the theory upon which this process is based is altogether speculative, and is not fully sustained by the facts in the case. Originally all mollusks are presumed to have been symmetrical, and are assumed to have resembled very much in form the schematic creature we have already described. Now, for some reason, certain of the primitive mollusks, but not all of them, began to develop a larger visceral mass, which, continuing to enlarge throughout many generations, began finally to protrude above and form a hump on the dorsal side of the animal. This hump, containing the liver, a portion of the intestines, and the generative glands, as it increased in bulk became so much elevated that it could no longer maintain itself in an erect position over the body, but, impelled by its own [pg330] weight, began to sag over to one side. Such a process would of course tend to twist the digestive tract and bring the excretory opening constantly farther forward on one side. At the same time, this process would necessarily interfere, by the weight of the overhanging viscera and the shell covering the mass, with the development of the branchiæ lying upon that side and now crowded under the visceral mass. Thus, according to this theory, this process has continued until, in the modern gasteropod, the digestive tract has been bent upon itself and twisted from a straight course into a curve of almost 180°, bringing the excretory opening near or just over the head, where it empties into the mantle cavity; while the original right gill, by the same movement, has been brought to a position on the left side of the head, forward of the heart, the original left gill having been crowded out, eventually to atrophy and disappear. The same torsion is found in the nerve-cords; the heart, situated at about the pivotal position of this twisting process, has turned about upon itself, leaving an auricle in front of the ventricle, one auricle, like one of the gills, having been lost. The mantle portion covering the visceral hump naturally continued to secrete its shell, though always in conformity with the change, the result being the familiar spiral form of the usual gasteropod shell.
This theory may not be satisfactory, but the asymmetry of gasteropods is a problem to be solved, and a more interesting line of biological investigation could not be found.
Let us now take a good example of a gasteropod and locate its various organs; at the same time we may use the occasion to refer to more important modifications of these organs which will be encountered later in the various genera. The most available gasteropod on the east coast of the United States for this purpose, is Fulgur, both on account of its large size and its abundance. Buccinum may be used if the student is north of Cape Cod and therefore unable to secure a good living specimen of Fulgur; the anatomical differences between the two are slight.
Note the siphon protruding forward from a notch in the shell. This consists merely in an elongation of a fold of the mantle, which is held in a manner to constitute a tube, through which [pg331] the animal draws in water to supply the gills, lying just back of the siphon and concealed under the mantle in the cavity already spoken of as the mantle cavity. With the exception of that portion of the mantle which constitutes the siphon, no part of its margin, it will be observed, can be seen, the edge of the shell completely hiding it from view.
In some genera the siphon is exceedingly long—in fact, longer than the body of the animal. In these cases the mollusk is generally fond of burying itself in the sand and maintaining communication with the world above only by means of this long, fleshy tube. In some genera the siphon is protected by an elongation of the shell, as in Fasciolaria (Plate LXXVI.). An extreme example of this is given in the figure on page 343, which represents a species of Murex, where the siphon is not only protected by the shell, but the shell itself is further protected by the double row of sharp spines with which it is beset.
There are some genera which possess no siphons at all, in all of which cases there is no notch in the shell opening, the aperture being round, as in Natica and in Littorina (pages 368, 371). The mollusks of this type make a fold in the forward part of the mantle which serves as a siphon. Such a muscular folding in the mantle would be technically called a functional siphon.
On the posterior dorsal side of the foot is attached a horny plate, of concentric structure about a central or subcentral point or nucleus. This is called the operculum, and serves to close the entrance of the shell when the animal is withdrawn, the object no doubt being protective. The opercula vary to a great extent in the different genera; in some cases they are calcareous, in others cartilaginous. When the aperture of the shell is very large it often happens that the operculum does not entirely close the opening, as in Fulgur, but in Buccinum it is a close fit. The operculum is entirely wanting in the Pulmonata, one of the [pg332] large orders of the Gasteropoda. The figures represent some of the commoner types of opercula.
The opercula have been utilized a great deal in the classification of the Gasteropoda, and especially in some groups that have become terrestrial in habit. Often, when shell-characters utterly fail to give a clue to the proper generic position of a new or rare mollusk, the operculum may be relied upon to indicate it.
Two tentacles project like fleshy horns from the sides of the head. These are not retractile, as in the land-snails, and their office appears to be that of sensory organs. In both Fulgur and Buccinum the tentacles are comparatively short and stout, and have expanded bases; in many other genera they are long and slender. Almost without exception the Gasteropoda possess tentacles. [pg333]
In Fulgur the eyes are situated near the bases and upon the outer sides of the tentacles; in Buccinum they are extremely small and are mounted about midway between the bases and the tips of the tentacles. This is the usual position for the eyes in the marine Gasteropoda. Only in the pulmonates, or land-snails, are the eyes placed at the summit of the tentacles. Sight does not appear to be a very important sense in the gasteropods, although the eye in some instances becomes a highly developed organ. A number of forms that have the habit of burrowing deep into the sand or mud are quite destitute of visual organs, and certain cave-dwelling land-shells have entirely lost the sense of vision.
The mouth will easily be seen in both Fulgur and Buccinum. It is a small slit with slightly thickened, fleshy lips, and is placed on the lower forward portion of the head. In the case of both of these genera the mouth is situated at the end of a proboscis. It will be noted that Fulgur has a comparatively long and stout proboscis, or snout, which is not retractile. Buccinum has a smaller one. It is fairly safe to assume that all gasteropods that possess a proboscis are carnivorous. This organ is greatly developed in some genera, where it actually exceeds in length the rest of the animal. In such instances, no doubt, the creature is enabled to reach the interior portions of the shells of its victims and to devour every vestige of their flesh, for the mouth is most conveniently placed just at the end of this long, trunk-like organ. The genera which have no proboscis are, almost without exception, vegetable feeders.
We must now remove the animal from its shell. It is well to boil it first, for this does away with the annoying mucous secretion that is freely exuded by glands in the foot when the creature is roughly handled. Alcoholic specimens are apt to be much contracted and hardened, and are therefore less satisfactory as subjects for anatomical study. If it is not convenient to boil the [pg334] specimen, break away the shell with a hammer, using care not to lacerate the soft, fleshy portion within; when this is accomplished, wash the animal carefully to remove the slimy exudation. Note the "visceral hump," which is spiral, and which formerly occupied the upper-whorl portion of the shell. The thin skin covering it is the mantle, which below is greatly thickened and free, lying about the foot like a heavy fleshy flap.
The mantle-edge in both of these examples is simple; that is to say, it possesses no fringe of tentacles, nor is it supplemented by extra processes, characters which mark many genera of marine Gasteropoda. In Fulgur and Buccinum the mantle-edge does not protrude below the edge of the shell; but in many genera, especially those which possess smooth, glossy shells, like the cowries (Cypræa) and the graceful Oliva, the mantle is proportionately very much larger. In these two genera, when the animal is extended, as in crawling about the sand, the mantle curves upward and incloses a large portion of the shell itself. Indeed, in some genera the shell is almost entirely concealed by this extension of the mantle (Sigaretus, Natica, etc.).
When the shell is removed, the folding of the mantle which constitutes the siphon can be plainly seen. The office of the siphon has already been referred to, also the fact that the presence of a siphon in the gasteropod mollusk may always be determined by merely glancing at the shell alone, for a notch at the base of the aperture indicates the place through which the siphon passed. In Buccinum this is merely a notch, but in Fulgur the siphonal canal of the shell is much longer. Just why the long siphons of some mollusks should be naked and exposed to danger, while others are so carefully protected by elongated, portions of the shell, is a mystery, but nature is full of such contradictions.
The foot is long, broad, and flat on the under side, like a disk. The variations in the gasteropod foot are almost infinite. [pg335] In some of the species it is amazingly large and powerful, as in Polynices (Natica), and in most of the forms which live along sandy beaches. These are sometimes provided in front with a wedge-shaped process called the propodium, which serves admirably as a plow to push aside the heavy wet sand through which the animal forces its way. In Nassa, which is so common all along our coasts, the foot has two terminal appendages or points behind.
There is no single feature of mollusks more important than the foot, for upon the many modifications of this organ the various molluscan classes are founded. The name Gasteropoda means "stomach-foot," the latter organ being merely a thickening of [pg336] the ventral surface of the animal, giving to the creature, when active, the appearance of crawling upon its stomach.
If a living gasteropod is placed in a jar of sea-water and left undisturbed it will soon crawl up the side of the glass and thus afford an excellent view of the extended under surface of its foot. The majority of the gasteropods are lethargic and slow in movement, but others will be found to be exceedingly lively and able to creep up the sides of the jar and make their escape in a surprisingly short time. Further modifications of the foot will be mentioned later.
Now let us place the animal with the foot below and the head facing us. The spiral visceral hump will point back and away from the observer. The mantle covering the visceral portion is thin and translucent like an ordinary skin, but at the base of the visceral mass the mantle suddenly thickens very greatly, and spreads over the dorsal portion of the foot or the body like a cloak. The space between the body and the loose-lying mantle is called the mantle cavity, and within this area are to be found several important external organs. To find these it is well to cut the mantle flap in a straight line, beginning just back of the head and ending at the point where the mantle is attached to the body. This will most easily be done with scissors. Having thus divided the mantle in front, throw back the two flaps. On the right-hand side (left of the animal), and attached to the inner side of the mantle thus exposed, will be seen the branchiæ, or gills.
They consist of a row of flattened filaments bound together at the base like the leaves of a book. The blood is conveyed to these gills by a large vein, and is then forced through the thinly walled filaments, being thus brought into close contact with the water, to which it releases its carbon dioxide, and from which it receives the life-giving oxygen. In certain gasteropods there are two sets of gills, one placed upon each side of the body [pg338] (as in the ideal mollusk); but the presence of two sets of gills only indicates, in these gasteropods, that the twisting process of the viscera, already alluded to as so remarkable a feature of this class, has not proceeded far enough to destroy the functional activity of one of the gills.