Chapter XIII: Part II: Marine Invertebrates (8)

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]

TABLE SHOWING THE CLASSIFICATION OF THE GASTEROPODS AND SCAPHOPODS DESCRIBED IN THIS CHAPTER

Phylum =MOLLUSCA=

Class =GASTEROPODA=

Order =OPISTHOBRANCHIATA=

Suborder =TECTIBRANCHIATA=

Families Genera Species

BULLIDAE ^Bulla^ ^B. (Haminea) solitaria^
^B. occidentalis^
^B. nebulosa^

APLYSIIDAE ^Aplysia^ ^A. Wilcoxii^

Suborder =NUDIBRANCHIATA=

DENDRONOTIDAE ^Dendronotus^ ^D. arborescens^

AEOLIDIDAE ^AEolis^ ^AE. papillosa^

DORIDIDAE ^Doris^ ^D. bilamellata^

Order =PROSOBRANCHIATA=

Suborder =DIATOCARDIA=

Families Genera Species

ACMAEIDAE ^Acmaea^ ^A. testudinalis^
^A. testudinalis,^
Var. ^alveus^
^A. mitra^
^A. patina^
^Lottia^ ^L. gigantea^

FISSURELLIDAE ^Fissurella^ ^F. alternata^
^F. barbadensis^
^F. (Glyphis) aspersa^
^Lucapina^ ^L. crenulata^

HALIOTIDAE ^Haliotis^ ^H. splendens^
^H. rufescens^
^H. cracherodii^

TROCHIDAE ^Margarita^ ^M. cinerea^
^M. helicina^
^M. undulata^

^Calliostoma^ ^C. occidentale^
^C. jujubinum^
^C. annulatum^
^C. canaliculatum^
^C. costatum^

^Chlorostoma^ ^C. funebrale^
^C. brunneum^

^Trochiscus^ ^T. norrisi^
^Livona^ ^L. pica^

TURBINIDAE ^Turbo^ ^T. castaneus^
Var. ^crenulatus^

NERITIDAE ^Nerita^ ^N. peleronta^
^N. tessellata^
^N. versicolor^

^Neritina^ ^N. reclivata^
^N. viridis^

Suborder =MONOTOCARDIA=

JANTHINIDAE ^Janthina^ ^J. fragilis^

SCALIDAE ^Scala^ ^S. lineata^
^S. multistriata^
^S. groenlandica^
^S. angulata^

NATICIDAE ^Polynices^ ^P. (Lunatia) heros^
^P. (Lunatia) triseriata^
^P. (Lunatia) lewisii^
^P. (Neverita) duplicata^
^P. (Neverita) recluziana^
^Natica^ ^N. clausa^
^N. canrena^
^Sigaretus^ ^S. perspectivus^

CAPULIDAE ^Crepidula^ ^C. fornicata^
^C. plana^
^C. aculeata^
^Crucibulum^ ^C. striatum^
^C. spinosum^

LITTORINIDAE ^Littorina^ ^L. litorea^
^L. rudis^
^L. palliata^
^L. irrorata^
^L. angulifera^
^L. scutulata^
^L. planaxis^
^Lacuna^ ^L. vincta^
^Tectarius^ ^T. muricatus^
^T. nodulosus^

CERITHIIDAE ^Cerithium^ ^C. floridanum^
^C. muscarum^
^C. minimum^
^Bittium^ ^B. nigrum^
^Cerithidea^ ^C. scalariformis^
^C. sacrata^

VERMETIDAE ^Vermicularia^ ^V. spirata^

STROMBIDAE ^Strombus^ ^S. pugilis^
^S. gigas^

CYPRAEIDAE ^Cypraea^ ^C. exanthema^
^C. spurca^
^C. spadacea^
^Trivia^ ^T. pediculus^
^T. quadripunctata^
^T. californica^
^T. solandri^

DOLIIDAE ^Dolium^ ^D. galea^
^D. perdix^
^Pyrula^ ^P. papyratia^

CASSIDIDAE ^Cassis^ ^C. tuberosa^
^C. cameo^
^C. testiculus^
^C. inflata^

Families Genera Species

MURICIDAE Subfamily Muricinae
^Murex^ ^M. rufus^
^M. pomum^
^Trophon^ ^T. clathratus^
^Urosalpinx^ ^U. cinerea^
^Eupleura^ ^E. caudata^
^Pteronotus^ ^P. festivus^
^Ocinebra^ ^O. poulsoni^
^O. lurida^
^O. interfossa^
^Cerostoma^ ^C. nuttallii^
^C. foliatum^
MURICIDAE Subfamily Purpurinae
^Purpura^ ^P. lapillus^
^P. patula^
^P. haemastoma^
^P. crispata^
^P. lima^
^P. saxicola^
^Monoceras^ ^M. lapilloides^
^M. engonatum^
^Chorus^ ^C. belcheri^

COLUMBELLIDAE ^Columbella^ ^C. mercatoria^
^C. (Anachis) avara^
^C. similis^
^C. (Astyris) lunata^
^C. (Amphissa) corrugata^
^C. (Astyris) gausapata^

NASSIDAE ^Nassa^ ^N. trivittata^
^N. obsoleta^
^N. vibex^
^N. fossata^
^N. perpinguis^
^N. tegula^
^N. mendica^
BUCCINIDAE ^Buccinum^ ^B. undatum^
^Chrysodomus^ ^C. decemcostatus^
^Sipho^ ^S. Stimpsoni^
^S. pygmaeus^
^Siphonalia^ ^S. kellettii^
^Tritonidea^ ^T. tincta^

TURBINELLIDAE ^Fulgur^ ^F. carica^
^F. canaliculata^
^F. perversa^
^F. pyrum^
^Melongena^ ^M. corona^

FASCIOLARIIDAE ^Fasciolaria^ ^F. gigantea^
^F. tulipa^
^F. distans^

VOLUTIDAE ^Voluta^ ^V. junonia^

MARGINELLIDAE ^Marginella^ ^M. apicina^

OLIVIDAE ^Oliva^ ^O. literata^
^O. mutica^
^Olivella^ ^O. biplicata^
^O. boetica^

CONIDAE ^Conus^ ^C. floridanus^
^C. pealii^

CANCELLARIIDAE ^Cancellaria^ ^C. reticulata^

Class =SCAPHOPODA=

DENTALIDAE ^Dentalium^ ^D. dentale^
^D. pretiosum^
^D. hexagonum^
^Entalis^ ^E. striolata^

[pg328]

CLASS =GASTEROPODA=

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 branchiae, 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 branchiae 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 deg., 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.

TENTACLES

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]

EYES

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.

MOUTH AND PROBOSCIS

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.

MANTLE

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 (/Cypraea/) 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 GASTEROPOD FOOT

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.

vigatus/, showing excessive development of the propodium (/pr./) and metapodium (/met./) in a mollusk living in sand (the shell, which covers only the liver and adjacent parts, has been removed): /l/, liver; /s. ap./, aperture of proboscis, here deflected from the median line; /t/, /t/, tentacles; /f/, foot.]

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.

THE MANTLE CAVITY

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 branchiae, or gills.

THE BRANCHIAE, 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.

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 /Littorinidae/, 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.

OSPHRADIUM

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]

RADULA

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 radulae 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 radulae of mollusks are capable of being grouped by their general characteristics, yet in no two species of mollusks are the radulae quite identical. [pg341]

VITAL ORGANS, STOMACH, LIVER, RENAL GLANDS, ETC.

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 cooeperate with the latter in the generative functions.

HEART AND VASCULAR SYSTEM

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.

THE GASTEROPOD SHELL

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 /Acmaea 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 striae/, /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.

/Acmaea 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 /Acmaea/ is an ancestral type just because the simple character of the shell is suggestive of the model chosen to represent a schematic mollusk; anatomically /Acmaea/ 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/.

CLASSIFICATION OF GASTEROPODS

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 /Cyclostomatidae/) 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.

ORDER =OPISTHOBRANCHIATA=

SUBORDER ~TECTIBRANCHIATA~

To avoid confusion, it is well to keep the systematic table in view:

Class Orders Suborders

=GASTEROPODA= =OPISTHOBRANCHIATA= =TECTIBRANCHIATA=
=NUDIBRANCHIATA=
=PROSOBRANCHIATA=
=PULMONATA=

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 /Bullidae/. 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 [pg351] "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.

SUBORDER ~NUDIBRANCHIATA~

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, [pg352] 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 algae 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 algae, 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^