Chapter XVI: Part II: Marine Invertebrates (11)

^F. gigantea.^ This is said to be the largest gasteropod shell
known. It often attains a length of sixteen inches, and specimens
are reported to have been found measuring no less than two feet.
This huge mollusk has a symmetrically fusiform shell, with whorls
obtusely ridged and armed with large rounded nodes. The aperture
is reddish-brown within. The anterior canal is long and open, and
there are three "plaits," or folds, about the columella. The outer
lip is more or less sinuate, the color is fleshy-white, and the
shell is covered with a thick yellowish or chestnut epidermis. The
animal is of a brilliant scarlet color. (Plate LXXVI.)

^F. tulipa.^ An exceedingly common species of Floridian waters,
which may justly be classed among the "show shells" of the American
Atlantic fauna. It has a typically symmetrical, fusiform shape,
with gracefully rounded whorls, and is spirally but not deeply
grooved. The sutures are crenulated; the anterior canal is long;
there are three entering folds upon the columella; and the outer
lip is simple. The scheme of coloration is variable, but there are
two chief patterns, which are usually followed, though they often
are considerably modified. One is a light-bluish body, variegated
with chestnut or olive blotches, which are more extensive upon
the spire than upon the body-whorl; the other is a pinkish body,
encircled with revolving black lines. The specimens of /F. tulipa/,
which are found so abundantly in the shallow bays and estuaries of
the Florida coast, are, for some reason, never brightly colored, as
are those taken in the open sea. The length is about four to six
inches. (Plate LXXVI.)

^F. distans.^ A form almost identical with that last described. It
is much smaller, varying from two to three inches in length, lacks
the crenulations of the sutures, and is perfectly smooth, save for
a few spiral ridges about the base. The color is bluish-white, with
clouded longitudinal [pg398] streaks of olive. Encircling the
shell are from four to seven black lines. (Plate LXXVI.)

These three species of /Fasciolaria/ often occur associated upon
sand-banks which are exposed at unusually low tides. It requires
some force to dislodge the large /F. gigantea/ from its bed of
sand. When the apex is discovered projecting an inch or less from
the bottom, the collector must scrape away the sand down to the
body-whorl, and then secure a good hold upon the spire and pull
hard. The stupid animal will not at once withdraw into his shell,
but will hang protruded from the aperture like a great piece of
vividly red meat. To extract any of these larger fasciolarias from
its shell, it is necessary to boil it for ten or fifteen minutes.
Large specimens of /F. gigantea/ should remain in the pot half an
hour. The heavy corneous epidermis of /F. gigantea/ will dry and
crumble away unless it is rubbed with vaseline about once a year.

FAMILY ~VOLUTIDAE~

GENUS ^Voluta^

The =Volutidae=, like the cowries, are "aristocratic" shells. As was stated above, there are many collectors who have become so fascinated by the shells of a few of these "first families of the /Mollusca/" that they make great efforts and spend no small sums in seeking to complete their cabinet series of the rare and more beautiful forms. The /Volutidae/ certainly have very beautiful shells, and they are for the most part exceedingly rare. The reason of their rarity has no doubt something to do with their habits. They are probably largely dwellers upon rocky ground, where the dredge cannot reach them. There is but one species of /Voluta/ upon the American coasts which is available to the shore-collector, and this is the well-known and much-sought-after ^V. junonia^. The animal of /Voluta/ has a very broad foot, and a head curiously divided into two lobes, upon which are placed sessile eyes. A remarkable feature of the genus is the lack of an operculum. [pg399]

^V. junonia.^ This volute has never been taken alive in shallow
water near the shore, but is occasionally found cast upon the
beach after violent storms. Sanibel Island is the best locality.
/V. junonia/ has an obtuse apex, a long aperture, and a shell
from three to three and a half inches in length. The color is
cream-white, painted with rows of large deep-red spots. In his
"American Marine Shells," published nearly thirty years ago, Mr.
Tryon says: "This is the most rare and valuable American marine
shell; specimens in good condition sell for from fifty to one
hundred dollars." A good specimen is probably not worth so much
to-day as when Mr. Tryon wrote, but it is at least worth a careful
survey of a Floridian beach after a storm.

FAMILY ~MARGINELLIDAE~

GENUS ^Marginella^

This is a family of exquisite little shells. In point of beauty they would rival the volutes were they not so much handicapped by their small size. The largest /Marginella/ is not more than an inch and a half in length, while the great majority of its two hundred tropical species do not exceed one half an inch. The animal is very similar to that of /Voluta/; indeed, the marginellas were until recently included in the /Volutidae/. The shell is also suggestive of /Voluta/. It is porcelanous, highly polished, and shining like all shells which are covered by a portion of the mantle when the animal is extended. Forty-seven species of /Marginella/ are recorded from the Atlantic coast of the United States. The marginellas frequent sandy or muddy sheltered spots, and may be collected between tide-marks. In California there are several species, which are so minute that it seems hardly worth while to attempt a description of them. They are white and very inconspicuous little shells. One is tempted, however, to speak of some of the fine marginellas of the Bahamas. The animals are quite as beautifully marked as the shells themselves, while the shell of one species is used in jewelry on account of its pearly luster suffused with the most delicate pink.

^M. apicina.^ The most abundant Floridian species of this genus. It
is not more than one third of an inch long, with a depressed spire,
a large body-whorl, a long aperture nearly equaling the entire
length of the shell, a thickened outer lip, and four very prominent
plaits on the columella. The surface is smooth. The white, enlarged
outer lip is glazed or highly polished, and bluish or brown in
color, with red spots. (Plate LXXVII.) [pg400]

FAMILY ~OLIVIDAE~

GENUS ^Oliva^

The genus /Oliva/ is one of the favorites among collectors. It comprises smooth, highly polished, porcelanous, and oval shells with a deeply notched, long, narrow aperture. The columellar lip is usually heavily calloused and ornamented with oblique folds. The animal's foot is very large, and extends laterally into lobes which curve back over the shell. In front the propodium is very large, forming, as in /Natica/ and /Polynices/, a sort of plow. The mantle lobes almost cover the shell when extended.

^O. literata.^ The only American example out of some sixty known
species. It is found in great abundance in sandy tide-pools along
the west Florida shores. It ranges from Hatteras to the West
Indies, and may be looked for in stations similar to those of the
naticas. It is exceedingly active, crawling rapidly over the sand
or burying itself very quickly out of sight, and is carnivorous.
The shell is highly polished, about two inches long, and pale
yellowish-white in color, covered with longitudinal, angulated,
or zigzag lines of chestnut. The spire is short, the sutures
channeled, and the columella calloused with white enamel, and
obliquely striated. (Plate LXXVII.)

GENUS ^Olivella^

The genus /Olivella/ resembles /Oliva/ very closely in conchological characters, but the shells are, with few exceptions, exceedingly small, and usually have a somewhat more elongated spire than is the case in the genus /Oliva/ proper. The animal of /Olivella/ seems to be a degenerate, for it possesses neither tentacles nor eyes. In habit and station it resembles /Oliva/.

^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.

^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 ~CONIDAE~

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 /Cypraea/, 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 L43 ($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 ~CANCELLARIIDAE~

GENUS ^Cancellaria^

The last family of the /Gasteropoda/ to be considered is the /Cancellariidae/, 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.)

CLASS =SCAPHOPODA=

FAMILY ~DENTALIDAE~

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 striae; 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 /Dentalidae/ 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 /Dentalidae/ 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 /Dentalidae/ 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]

TABLE OF THE COMMONER NORTH AMERICAN PELECYPODS DESCRIBED IN THIS CHAPTER

Class =PELECYPODA=

Order =PROTOBRANCHIATA=

Families Genera Species

NUCULIDAE ^Nucula^ ^N. proxima^
^Leda^ ^L. tenuisulcata^
^Yoldia^ ^Y. limatula^
^Y. thraciaeformis^
^Y. sapotilla^
SOLENOMYIDAE ^Solenomya^ ^S. velum^
^S. borealis^

Order =FILIBRANCHIATA=

ANOMIIDAE ^Anomia^ ^A. simplex^
^A. aculeata^
^A. lampe^
^Placunanomia^ ^P. macrochisma^

ARCIDAE ^Arca^ ^A. ponderosa^
^A. noae^
^A. pexata^

MYTILIDAE ^Mytilus^ ^M. edulis^
^M. hamatus^
^M. californicus^
^Modiola^ ^M. modiolus^
^M. plicatula^
^M. tulipa^
^M. recta^

Order =PSEUDOLAMELLIBRANCHIATA=

AVICULIDAE ^Avicula^ ^A. atlantica^
^Pinna^ ^P. muricata^
^P. seminuda^
^Perna^ ^P. ephippium^

OSTREIDAE ^Ostrea^ ^O. virginica^
^O. frons^
^O. lurida^

PECTINIDAE ^Pecten^ ^P. magellanicus^
^P. irradians^
^P. dislocatus^
^P. islandicus^
^P. aequisulcatus^
^P. hastatus^

Order =EULAMELLIBRANCHIATA=

CARDITIDAE ^Cardita^ ^C. borealis^
^C. floridana^

ASTARTIDAE ^Astarte^ ^A. castanea^
^A. undata^

CYPRINIDAE ^Arctica^ ^A. islandica^

LUCINIDAE ^Lucina^ ^L. tigrina^
^L. floridana^
^L. pennsylvanica^
^L. dentata^
^L. californica^
^L. nuttallii^
^Loripes^ ^L. edentula^

TELLINIDAE ^Tellina^ ^T. radiata^
^T. alternata^
^T. bodegensis^
^T. tenera^
^Macoma^ ^M. secta^
^M. nasuta^
^M. baltica^
^M. proxima^
^M. tenta^

DONACIDAE ^Donax^ ^D. variabilis^
^D. californicus^

MACTRIDAE ^Mactra^ ^M. solidissima^
^M. similis^
^M. ovalis^
^M. lateralis^
^R[oe]ta^ ^R. canaliculata^

VENERIDAE ^Venus^ ^V. mercenaria^
Var. ^mortoni^
^V. cancellata^
^Cytherea^ ^C. (Callista) gigantea^
^C. (Callista) maculata^
^Dosinia^ ^D. discus^
^D. elegans^
^Tivela^ ^T. crassatelloides^
^Tapes^ ^T. staminea^
^T. laciniata^
^Chione^ ^C. succincta^
^C. fluctifraga^
^C. simillima^
^Saxidomus^ ^S. nuttallii^

PETRICOLIDAE ^Petricola^ ^P. pholadiformis^
^P. carditoides^

CARDIIDAE ^Cardium^ ^C. magnum^
^C. isocardia^
^C. laevigatum^
^C. serratum^
^C. mortoni^
^C. substriatum^
^C. corbis^
^C. quadrigenarium^
^C. elatum^
^C. islandicum^
^C. pinnulatum^

MYIDAE ^Mya^ ^M. arenaria^

SOLENIDAE ^Ensis^ ^E. directus^
^Solen^ ^S. viridis^
^S. sicarius^
^S. rosaceus^
^Tagelus^ ^T. gibbus^

GLYCIMERIDAE ^Glycimeris^ ^G. generosa^

PHOLADIDAE ^Pholas^ ^P. costata^
^P. truncata^
^P. californica^
^Zirph[oe]a^ ^Z. crispata^

TEREDINIDAE ^Teredo^ ^T. navalis^

PANDORIDAE ^Pandora^ ^P. trilineata^

[pg408]

[pg409]

CLASS =PELECYPODA=

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

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 papillae, 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.

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.

MANTLE FUSION

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.

THE BRANCHIAE OR GILLS

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 aerated 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 lamellae 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

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 branchiae 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.

THE PELECYPOD SHELL

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 /Veneridae/ 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.

CLASSIFICATION OF THE PELECYPODA

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 branchiae, 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/.

The first includes the simpler type of gill as represented in the accompanying figure (A). Its filaments are short and not reflected. The second, represented in Fig. B, has the filaments long, reflected, and connected (each filament to its adjacent ones) by means of surface cilia. The third type of gill resembles the last except that the ends of the filaments of the outer gill are attached to the mantle, and the ends of the filaments of the inner gill are attached to the foot or visceral mass. The fourth type of gill is far more highly [pg421] developed. The adjacent filaments are connected, not by cilia, but by vascular channels, and the depending and recurved portions of each filament are also connected in a manner presumed to admit of the passage of blood. This gives to the organ the appearance of basketwork or network. The ends of the filaments are attached to the mantle and to the visceral mass, as in the last order, forming cavities or chambers above (see Fig. C). This order includes the great majority of the /Pelecypoda/.

The fifth and last order is confined to but two rather obscure families, in which the gill-development is carried to the point of substitution of muscular partitions which form a separate chamber connected with the mantle cavity by a narrow slit, the surface of the chamber having respiratory functions (Fig. D).

The collector of living marine objects who has become fascinated by their beauty and who derives pleasure from examining the greater beauty of their structures will find a rich field for research in the pelecypod gills. But unless he has time at his disposal and some skill with a microscope, he would do well not to venture too far into this alluring but difficult and vexatious subject.

Our coastal waters are fairly rich in bivalves, but the most strikingly handsome species of this class--and there are many of them--are inhabitants of warmer seas. Of the great number of bivalves to be found upon our own shores we can only mention briefly some of the commonest species.

ORDER =PROTOBRANCHIATA=

FAMILY ~NUCULIDAE~

In this family the gills are of the simplest and most primitive type. The mantle-edges are entirely open along the ventral margin, but they are united posteriorly in some of the /Nuculidae/ to form two short siphons. The palps are exceedingly large. The foot is also large, and by its disk-like surface is adapted to creeping, much as is a gasteropod foot; its edges are often crenulated or scalloped, and there is no trace of a byssus. The shells are [pg422] equivalve, and their most prominent feature is the hinge, which is beset with sharp, comb-like teeth in two straight rows, one upon each side of a central pit, a space reserved for the ligament.

GENUS ^Nucula^

^N. proxima.^ The commonest of several species of the genus found
along the New England coast. It is oblique, with a light-olive
epidermis, nacreous interior, and finely crenulated margins. Its
length is about one quarter of an inch. The animal has no siphons.
It lives in either muddy or pebbly stations near the shore, and
probably exists in countless thousands in all the bays and harbors
of the New England coast.

GENUS ^Leda^

^L. tenuisulcata.^ This species has a longer shell than the
preceding; it is more produced behind, and is narrowed into a
blunt, slightly gaping point. The epidermis is light greenish, and
the shell has a pearly luster within. The outer surface of the
shell is concentrically grooved; the beaks are smooth. The foot is
disk-shaped, and the animal is provided with small siphons. The
length of this shell is about one inch. Its station is the muddy
bottom in shoal water on the New England coast.

GENUS ^Yoldia^

^Y. limatula.^ This species has the same sort of toothed hinge
exhibited in the two preceding species, but the shell is
considerably larger (two inches), with a smooth greenish glazed
epidermis. The beaks are nearly central. The interior of the shell
is light bluish and pearly. The animal has two slender, short,
united siphons and a disk-shaped foot with simple margins. This
species can make excellent use of its foot, for it crawls with
rapidity, and also can execute leaps in an astonishing manner.
Found in muddy stations in shoal water on the New England coast.

^Y. thraciaeformis.^ A much larger species, found on the New England
coast. It may be distinguished by a rib-like wave extending
obliquely from the umbones to the ventral margin of the shell. It
measures over [pg423] two and a half inches in length and one and
a half inches in height, and is dark olive-green. The foot is like
that of /Y. limatula/. The labial palps are exceedingly large.

^Y. sapotilla.^ The shell is ovate, prolonged on one side, thin,
fragile, translucent, and covered with a thin, glossy epidermis,
greenish in color. The anterior half is semi-oval, the posterior
portion narrowed and compressed; within it is pearly-white, with a
triangular cartilage cavity, and sixteen to eighteen pointed teeth
on each side. In length it is little less than an inch. Found from
Long Island Sound northward.

FAMILY ~SOLENOMYIDAE~

GENUS ^Solenomya^

Two species of this genus, which are greatly prized by collectors on account of the extraordinary development of the epidermis of the shell, occur in New England waters. The strong corneous periostracum of a deep chestnut-color projects considerably beyond the margins of the valves; the shells are exceedingly thin and fragile, and are marked with radiating lines. The two species are easily distinguished by their difference in size. ^S. velum^ is about one inch long and one half of an inch high. ^S. borealis^, of arctic range, is quite twice that size. Their anatomy is similar in essentials to that of /Yoldia/. These species live near shore in sandy bottoms, and are occasionally found upon the beach between tides. Chelsea Beach, just north of Boston, is an often-quoted locality.

ORDER =FILIBRANCHIATA=

There are several families of this order which are well represented in both the Atlantic and the Pacific waters of the United States. The characteristic feature of this group of bivalves is the filamentous gill, that is, a gill with the filaments long, doubled back, and united to each other only by ciliary junctures. These [pg424] junctures are so slight that they break readily when a specimen of a gill is handled, leaving the filaments free. The presence of a byssal gland in the foot, and often a well-developed byssus, is another characteristic of this order.

FAMILY ~ANOMIIDAE~

GENUS ^Anomia^

This is a family of peculiar and highly specialized forms. /Anomia/ has an irregularly rounded shell, with one convex and one flat or concave valve. There is no regular hinge or well-defined hinge-margin, but a raised fossette, or cartilage plate, occupies a position at the top of the valves. In the flat valve there is a large oblong hole just under the apex, through which projects a calcified byssus, by means of which the animal secures itself to oysters, dead shells, stones, or any solid object. The anomias, which have become stationary in habit, have practically lost their foot. The gills are very large and curved, while all the organs seem to be abnormally placed on account of the huge byssus and byssal muscle. For the byssus to pass, as it does, through a specially prepared hole in one of the valves is an extraordinary departure from the conventional types of byssiferous species. The heart is not traversed by the intestine. Altogether, then, /Anomia/ is a very curious genus.

^A. simplex.^ The commoner large form of New England. It varies
from one to three inches in diameter, is exceedingly irregular
in shape, and its surface is variously undulated and plaited in
accordance with the surface of the object to which it is attached.
Thousands of these valves, disjointed and separated, are cast upon
the beaches all along our Atlantic coast. They are light green to
salmon- or copper-color, generally fragile and scaly, and have a
peculiarly dulled (as though greased) nacre. [pg425]

^A. aculeata^ is much smaller than the last species, and the
surface of the convex valve is roughened by prickly scales. The
smaller valve is very thin, with an almost circular aperture for
the byssus. Its diameter is about half an inch. Found attached to
stones, etc., about the holdfasts of /Fucus/.

^A. lampe^, a California species ranging more to the south, is
small (half an inch), and greatly resembles the east-coast /A.
aculeata/, save as regards the smooth surface of the former.

GENUS ^Placunanomia^

^P. macrochisma.^ This species occurs on the Californian coast. It
has the same sort of calcified byssus as /Anomia/, and also the
hole to accommodate it in the smaller valve. The scar formed by the
muscle which controls the byssus is curiously rayed in this genus.
The normal shape of the shell is circular; it is pearly within, and
of a greenish tinge without. Two to four inches in diameter. (Plate
LXXVIII.)

FAMILY ~ARCIDAE~

GENUS ^Arca^

The /Arcidae/ may always be distinguished by the long row of comb-like teeth upon the hinge, the total want of siphons (a fact which may be discovered in the shell alone by its lack of a pallial sinus), and the solid trapezoidal or round shell, with its tendency to strong ribs radiating from the umbones. The periostracum, or epidermis, is heavy and often velvety or even hairy. The animal of /Arca/ has the gills placed in an oblique position; the foot is large and strongly developed. No trace of mantle fusion exists. The mantle-edges bear a row of composite eyes. The gill-filaments are entirely free, there being even no ciliary junctions.

^A. pexata.^ An exceedingly common species in Long Island Sound and
on the coast of New Jersey. The shell is oblong, with prominent
beaks directed forward. The hinge-teeth are arranged in a row
posterior to the beaks, while just under the beaks are a number of
irregular cartilage-pits. About thirty-two to thirty-six radiating
ribs ornament the shell. The inner margins of the shell are rather
deeply scalloped; the epidermis is thick, shaggy, and dark
[pg426] brown. Length two to two and a quarter inches; height two
inches. /A. pexata/ is one of the few pelecypods which have red
blood, a fact which accounts for its popular name of "bloody clam."

^A. ponderosa.^ The most prominent /Arca/ upon our Atlantic
coast, especially south of Virginia. The beaks of this shell are
very prominent, and do not approach each other very closely on
account of the wide dorsal margin of the shells, the large space
between the beaks being occupied by an external ligament. The
hinge-teeth are smaller in the middle. There are from twenty-five
to twenty-eight ribs; the ventral margin of the valves is somewhat
contracted in the middle. Length two and a half inches; height two
inches. Very heavy and solid. This exceedingly common species is
often cast upon the beaches south of Hatteras in numbers beyond
computation. In life it is covered with a heavy, coarse, velvety
epidermis, almost jet-black in color. (Plate LXXVIII.)

^A. noae.^ The well-known "Noah's-ark" shell, a common species
along the shores of the Southeastern States. It also occurs in
the Mediterranean. Unlike most arcas, it spins a byssus, by means
of which it attaches itself to the under surfaces of stones at
low water. The hinge-margin is perfectly straight and regularly
toothed. The beaks are high and are situated very far forward. The
dorsal margin of the valves is strikingly large, and is marked by
lattice-like grooves; it forms a concave surface over an inch in
width between the umbones. The shell is strongly ribbed without.
The ventral margin of the valves is sinuous and gapes slightly at a
central point for the accommodation of the byssus. (Plate LXXVIII.)

FAMILY ~MYTILIDAE~

GENERA ^Mytilus^, ^Modiola^

The /Mytilidae/ are a large family, including the mussels, so commonly found between tides upon all shores. The two ordinary genera /Mytilus/ and /Modiola/, which often occur associated together upon our eastern coast, may readily be distinguished one from the other by the position of their beaks. In the former genus the beak is terminal, that is, it is situated at the very tip-end of the shell; in the latter genus it is not quite terminal. Aside from this simple feature, the genera are practically the same, the animals being identical. /Mytilus/ and /Modiola/, commonly known as mussels, have acquired the stationary habit and are provided with a relatively small foot (in some species amounting to no more than a mere tubercle); but in place of a useful foot is a well-developed byssogenous gland, which secretes an [pg427] exceedingly tough, fibrous byssus. By means of this the animal may not only attach itself firmly to any sort of object, but may actually move about. Fibers of the byssus are thrown out, and, as though possessed of life, they seize upon objects in the direction in which /Mytilus/ desires to move, and by contraction of the muscles which control the byssus the animal is drawn forward, the operation being then repeated. It must be a very tedious method of travel. However, the /Mytilidae/ are not restless creatures. They pass their lives content to remain firmly fixed to some object, and their movements, if any, are within a very narrow circle. The animal has very long gills of the usual filamentous type. The mantle fuses at one point to form an anal siphon, the lower or branchial one being, therefore, functional. The palps are large. One curious feature of this group is the enormous extent of the genital glands, which actually appear to pervade the entire structure of the animal, gills, mantle, foot, and all. At times the whole being is a veritable receptacle for myriads of tiny yellowish eggs.

The /Mytilidae/ are of universal distribution, and are for the most part of littoral or shallow-water range. Some species are esteemed as an article of food, notably /Mytilus edulis/.

Mussel-culture has been profitably carried on in France for a long time, but the genus has not been utilized to any extent as food in this country. [pg428]

^Mytilus edulis^ is exceedingly abundant along the New England
coast. It lives in colonies, often between tides, in pebbly
stations among large rocks. It is always so fastened by its byssus
that the beaks of the valves point downward, and the ventral margin
projects above the sand or mud. The color of the epidermis in the
adult form is black or a deep blue-black; the shell proper is
violet. Hinge-teeth are wanting. Length two and a half inches. A
variety of this species, ^M. pellucidus^, is often found associated
with the typical form. It is brightly rayed in green and yellow.
/M. edulis/ occurs in Europe, where it is quite extensively used as
food. It also occurs (by importation) in San Francisco Bay.

^M. hamatus^ has a more southern range than has /M. edulis/, and
is found from the Chesapeake southward. Its shell is considerably
twisted just below the umbonal region, while its whole surface is
densely striated. Its color is dark fuscous. Length one to two
inches. It is found in great numbers attached to oysters by its
byssus. In Florida it is very common. (Plate LXXVIII.)

^M. californicus.^ A common west-coast species, which finds its
station among rocks exposed to the surf. It is about the same size
as /M. edulis/, but has a lighter brownish glossy epidermis and
several conspicuous, well-rounded, radiating ribs. The animal is
orange-colored.