C. magnum. The largest and finest "cockle" of the east coast of the United States. It is, indeed, one of the finest cockles in the world. The almost perfect heart-shape is striking. The posterior side is somewhat flatly depressed. The thirty-three to thirty-seven regularly disposed, broad, radiating ribs; the regularly crenulated margins; the yellowish-brown color garnished with transverse rows of chestnut or [pg454] purple lines or spots; the brownish-purple posterior area—all combine to make this large Cardium a beautiful species. Its length is four inches; height five and a quarter inches. It is a very abundant species on the open Florida beaches, where it is often left exposed and alive at very low tides. (Plate LXXXIV.)
C. isocardia. This also is a Floridian species, of elongated heart-shape and with radiating ribs. It is not more than half as large as the last-described species. The ribs are decorated with erect vaulting scales which are exaggerated portions of growth-lines. The beaks are smaller in proportion than in C. magnum. The shell is brown to straw-color, stained with purplish-brown without, and has a bright salmon-red or purplish-pink interior. The margins are deeply crenulated. The teeth are arranged as in the last-described species. It is very common on the beaches of western Florida. (Plate LXXXIV.)
C. serratum, C. lævigatum. These two species have a perfectly smooth surface, and inflated, globose, heart-shaped shells, and are of a creamy-white color, suffused with a yellowish, golden tint which suggests the color of butter. Some forms are shiny, and citron-yellow and pinkish toward the margins, whence the name of "peach-shell." Height one to two inches. Not uncommon upon Florida beaches. (Plate LXXXIV.)
C. mortoni. Another of the smooth forms of Cardium (placed in the subgenus Lævicardium). It occurs in Florida and also extends up the coast to Cape Cod, being very abundant in Long Island Sound. In the neighborhood of Martha's Vineyard and along the north shore of Long Island this very pretty little species has been reported as occurring in soft ground even above low-tide mark, near the mouths of creeks. It is enough to say of it that it is a smaller edition of C. lævigatum and may be further distinguished by a purple blotch on the posterior margin, just within the valves, the general color within being bright yellow. In young specimens, zigzag lines of dark fawn-color upon the pale-yellowish background of the smooth exterior surface of the shells are a noticeable feature. The largest specimens measure an inch in length and nearly the same in height. The long cirri upon the siphons are striking.
C. substriatum. A cockle of this inflated, smooth type, which strongly resembles the east-coast form, found upon the Pacific coast. The name indicates that it is not altogether smooth, a fact only revealed, however, by a magnifying-glass. It is about one half of an inch in length, and of a light drab-color, spotted and sometimes radially lined with yellowish-brown. Professor Keep likens this species in both shape and color to a sparrow's egg. (Plate LXXXIV.)
| PLATE LXXXIV. | |
|---|---|
| 1, Cardium magnum, much reduced. | 4, Cardium substriatum, enlarged. |
| 2, Cardium isocardia. | 5, Cardium corbis (young specimen). |
| 3, Cardium lævigatum. | 6, Glycimeris generosa. |
C. elatum. A veritable giant among the cardiums. It is found on the southern Californian coast, though rarely north of the Mexican border. It is of the smooth, glossy type belonging to the subgenus Lævicardium. It attains a diameter of six inches, and is of a creamy-yellow appearance.
C. corbis, C. quadrigenarium. These two Californian cockles are of the ribbed type displayed in the eastern C. magnum and C. isocardia. C. corbis is found in the northern Californian and Puget Sound region, [pg455] and C. quadrigenarium on the southern shores of California. The former is a full, round, heart-shaped shell with about thirty somewhat scaly ribs. The edge of the shell is deeply toothed; the color light brownish; the diameter from two to three inches. It preserves all the features of the genus as regards both shell and anatomical characters. The other species, C. quadrigenarium, very strongly resembles C. magnum in size and shape. It lacks the flattened area on the posterior portion of the shell which is characteristic of the east-coast species, and its uniform brownish-white coloration is less striking. It has about forty regularly spaced, radiating ribs, which are smooth upon the umbonal region, but elsewhere are armed with spiny processes. Diameter about six inches. (Plate LXXXIV.)
C. islandicum, C. pinnulatum. Both of these are cold-water species and occur along the New England coast. The former has a shell which varies from one half of an inch to two inches in length, and has from thirty-six to thirty-eight sharp, three-sided, radiating ribs, the furrows between them being rounded and wrinkled by growth-lines. A yellowish-brown epidermis covers the shell and bristles into a sort of fringe upon the sharp edges of the ribs. C. islandicum cannot be said to be a very common species in New England, although collectors have so reported it. Specimens from Massachusetts are not as large as those of more northern habitat. It does not occur south of Cape Cod. C. pinnulatum is one of the "small fry" among the cockles. Its largest diameter is less than one half of an inch; but what it lacks in size it seems to endeavor to make up in abundance, for it is scattered everywhere along the coast from New York northward. Specimens can nearly always be found in the stomachs of fishes, which, by the way, form an excellent hunting-ground for rare species of mollusks of small size. There are twenty-six slightly rounded ribs, with deep linear grooves between them. Upon the ribs, especially in the posterior portion of the shell, are arched scales, folded so as to appear like blunt spines. The species may easily be distinguished from the young of C. islandicum by the smaller number of ribs. The animal, C. pinnulatum, is said to be exceedingly lively and able to make rapid progress over gravelly bottom by executing sudden leaps. It has a long, recurved, very strong foot, and its movements are effected as described above. It is abundant at Bar Harbor. [pg456]
Genus Mya
In this family we encounter, rather suddenly, some new features not heretofore seen in the pelecypod structure. In the first place, the siphons are enormously large, are united, and are surrounded by a leathery epidermis. The mantle-edges are fused together along the entire ventral margin, except for a small slit through which the foot may project. Although the animal may withdraw the long siphons into its shell, yet they remain to a certain extent exposed, for the valves gape widely posteriorly, and only slightly less so anteriorly. In the economy of these forms the shell seems to play a less important part than it does in the Veneridæ, in the Tellinidæ, and generally in those families whose shells are strong and, closing firmly, afford the animal within a real protection. The shell of Mya (the principal genus) is thin, white, and of a softer chalky texture; it gapes widely "fore and aft," and has a loosely constructed hinge apparatus, consisting of an erect projecting tooth, which fits into a pit in the opposite valve.
M. arenaria. This is the common "soft-shell clam" of New England. Its range is from Cape Cod to Greenland and Great Britain. Upon the Maine coast it is very extensively gathered and sold to the Banks fishermen for bait. Its use as food for man is probably not very great, yet it is always to be seen on sale in the markets of New England coast towns. It cannot compare in flavor with Venus mercenaria, the "hard-shell clam" south of Cape Cod. M. arenaria lives between tides in muddy, sandy, pebbly, or even rocky ground, where it can find material in which it can burrow and hide itself. It lies just below the surface, with its siphons projected into the water. When the water recedes, Mya draws in its siphons and awaits the return of the tide, every now and then [pg457] ejecting a jet of water into the air. The shells vary considerably in size and thickness of valves. Large specimens are three and a half inches long and two inches high. Our forefathers were not always well posted upon the habits of mollusks, even though they may have relished them in chowders and believed their flesh to possess valuable curative properties. John Winthrop, in 1634, gave a list of useful American animals, in which he remarked: "Clam, white; their broth is most excellent in all intermitting fevers, consumption, etc. These clams feed only on sand." What marvelous digestive powers!
In this family are included the long, slender bivalves commonly known as "razor-shells." Every one who has been to the sea-shore has become familiar with these odd-looking mollusks, for their valves are always to be found upon every beach. Their station is upon sandy flats or bars more or less exposed at low tide. They burrow into the sand perpendicularly to a depth of two or three feet, remaining hidden most of the time. Occasionally a colony of them will be seen, each one projecting slightly from his burrow. If approached most cautiously some of them may be captured, but if the sand is jarred they all take fright and disappear in an instant. It is no easy matter to capture a "razor" when once he has taken warning, for he will dig down into the sand about as fast as one can follow with a spade. A good way to catch one is to approach his burrow carefully, and then plunge a spade obliquely down below him, thereby [pg458] cutting off his retreat. So tenaciously will they cling to the sand by expanding the muscular foot that the shell may be pulled entirely off the body before they will let go.
Genus Ensis
E. directus (Solen ensis, Ensis americanus). The common species upon the New England and Jersey coasts. The foot is long, and protrudes from one end of the long shell; it is also very strong, and capable of change at will into almost any form, from a pointed bulb to a flat disk. The siphons, which project from the opposite end of the shell, are short and are not united. The gills are long. The juxtaposition of these organs is at first confusing, because the greatly elongated form of the shell and the habits of the animal in burrowing vertically into the sand have caused the foot to be pushed farther away from the posterior portion of the shell and to protrude in a direction just opposite to the siphons. The shells are bent or slightly curved; they gape at both ends; and they are fitted with very small interlocking teeth at the upper corner of the posterior end. The color is white, but a yellowish or greenish glossy epidermis covers the entire test. Length about six inches. The manner in which the animal makes its rapid descent into the sand is very interesting. First extending the foot lengthwise into a point, it plunges it into the sand; then, by forcing water into the organ it expands it, thus pushing away the sand on all sides; and still further expanding the foot at the end into a disk, it secures an anchorage which enables it to draw down its shell. By quick repetition of this process Ensis can get out of sight in a remarkably sudden manner.
Genus Solen
S. viridis. A smaller species, about two inches in length, and with a nearly straight light-green shell. It is often abundant from New Jersey southward.
S. sicarius. The common Californian species of Northern range. It is only about two inches long, and is rounded anteriorly, but is chopped off squarely behind. It is slightly curved and white, and has a glossy brown epidermis.
S. rosaceus. Another species of Southern range, very similar in form to the last, but straighter and rosy-white in color. The epidermis is glossy brown. Length two inches. [pg459]
Genus Tagelus
T. gibbus. The extended range and shore station of this exceedingly abundant species will cause it to be among the first accessions to the cabinet of the collector south of Cape Cod. It burrows deep in sand and mud, leaving two small openings to the hole for the accommodation of its two excessively long siphons. As in Solen and Ensis, the foot is large and muscular, tongue-shaped, and capable of remarkable change of form and great freedom of movement. The long white siphons, separated from the base and each provided with orange-colored eyes (or, rather, a rudimentary sort of visual organs), are the most important feature. The apex of the hinge is not, as in Solen and Ensis, at the end of the shell, but is more conventionally placed near the middle; the cardinal teeth are very small, two upon each valve and interlocking; a flat, oblong, callous process serves as a fossette. The dorsal and ventral margins are parallel, and gape at both ends; the shell is white, and is covered with a dense yellowish epidermis, which, passing the ventral margin, appears to become continuous with the thickened mantle-edge. There is no other species north of Hatteras with which this could be confounded. The double entrance to the burrow, which may be discovered about low-tide mark, is unique. T. gibbus is a good species to examine for the crystalline stylet in the digestive tract.
Genus Glycimeris
G. generosa. A large shell of the Pacific coast, occurring most abundantly in Puget Sound waters, where it frequents muddy shallows. It is a remarkable pelecypod in respect to its siphons and the degree of mantle fusion. The appearance of the animal is that of a huge pair of [pg460] united siphons, protected laterally at their larger end by two white, widely gaping valves. The very small pedal opening in the line or mantle juncture indicates a very small foot. (Plate LXXXIV.)
There is a striking resemblance between the shells of this rock-boring family and those of the petricolas—a resemblance which consists principally in the texture of the shell and the tendency to different types of sculpturing upon the posterior and anterior surfaces of the valves.
Genus Pholas
In Pholas the anterior part of the hinge-margin is reflected back over the umbones, and a long rib-like tooth springs from under the umbonal region and curves out almost to the center of the valves. The shell gapes "fore and aft," and is thin, white, very hard, and brittle. The animal has a short, truncated foot and a small orifice in the mantle through which the foot may be projected. The siphons are long and are united almost to their ends. Dorsally, an exposed portion of the animal is protected by accessory calcareous plates. The habits of Pholas are very interesting. It is found in holes gouged out of solid rock or out of pieces of wood; sometimes, like Petricola, it may be found excavating in hard clay. As the anterior end of a Pholas shell is the larger in diameter, and is the end which is most deeply buried in the rock (the opening of the burrow being comparatively small), there was some mystery as to the way in which the creature manages to get inside its rocky dwelling. Observations in aquaria have shown that the young Pholas begins his process of home-building very early in life. The wearing away of the stone is effected by constantly turning the shell around, scraping with the anterior edges of the valves. A lodging-place having been secured, Pholas is obliged to work constantly to enlarge his burrow for the accommodation of his growing shell. The long siphons may project from the hole and draw in food and breath, but the animal is a prisoner for life. When Pholas has withdrawn his long siphons he is reasonably secure from his enemies, but [pg461] nevertheless thousands of them are destroyed by predatory starfishes, who know how to get at their victims. The restless crustaceans, ever on the outlook for a meal, often nip the siphons of the rock-dwellers, and a kind of worm sometimes attacks them, and, destroying the animal, proceeds to occupy the empty shell and burrow.
P. costata. This species has a wide geographical range, being found from Cape Cod to South America, but it is not abundant north of Hatteras. In Florida it burrows deep in sand as well as in wood or rock. On account of its white color and suggestive shape and sculpture, it has received the popular name of "angel's-wings." It grows to a length of seven or eight inches.
P. truncata. This has much the same range as the last, but is more commonly to be found in New England waters. It also burrows in any hard substance, or in mud above low-water mark. It is especially common in peat-banks.
P. californica. A Californian Pholas. The shell is about three inches in length, and resembles in all features and habits the Pholas of the east coast. Differences in the shell indicate that it is a distinct species.
Genus Zirphæa
Z. crispata. A species of Northern range, occurring but sparingly in New England, in hard clay or rocky burrows. It may be identified at once by the furrow which passes from the beaks across the valves to the lower edges of the shell and divides the surface of the valve into two areas. The anterior area is decorated with radiating toothed ribs; the posterior area is smooth. This conchological feature of the genus is very [pg462] curious; it may be observed also in several deep-sea genera and in certain extinct fossil forms. The length of this shell is about two inches. This species also occurs in California, where its favorite station is in hard tenacious clay. Length two to four inches.
Genus Teredo
T. navalis. This species is worm-like in form, but it has a small bivalve shell at the larger end, and near the anterior extremity two calcareous appendages called pallets, beyond which extend two siphons. Along the surface of the mantle is secreted a continuous shelly tube which lines the burrow. This mollusk, commonly called the "ship-worm," is exceedingly destructive, perforating with its burrows submerged timber and soon rendering it useless. Various means are taken to protect ships, the piles of wharves, buoys, and the like, against its ravages; copper sheathing, large-headed nails driven close together into the wood, verdigris paint, and so on, being used with more or less effect. Vast numbers of these animals enter the wood and burrow in various directions, but they never interfere with one another, a thin partition of wood always being left between adjacent burrows. How they effect the burrowing is not determined, but it is supposed to be by means of the pallets. Teredo does not, like the boring isopod Limnoria lignorum, feed upon the wood. Its food consists of microscopic organisms which are taken in through the incurrent siphon. In temperate waters T. navalis grows sometimes to the length of six inches; in tropical waters it attains the length of two feet.
There are three other species of Teredo and one of the genus Xylotrya on our Northern shores; T. navalis is, however, the most common and most destructive. In Southern waters there are many other forms of these boring mollusks. [pg463]
Genus Pandora
P. trilineata. A little New England shell, remarkable on account of its extreme flatness. When looking at this shell before opening it, one naturally wonders where the animal finds room to exist between two such disk-like valves. The animal is very thin, with largely united mantle-edges, widely separated feeble adductor muscles, and a fairly large tongue-shaped foot. The shell is nacreous, rounded anteriorly, and produced posteriorly into a sort of upturned tip which gapes to accommodate two little siphons. The dorsal hinge-margin is excavated and curved. Length about one inch. It is abundant at Cape Cod, in sandy stations, on oyster-beds, and is found from Maine to Florida. [pg464]
The Cephalopoda form a singularly isolated group, and are so superior in organization and intelligence to all other mollusks that it is difficult to believe that they are first cousins to the lethargic gasteropod and the simply constructed bivalve. But the class bears the stamp of its origin in a mantle, a radula, and a disposition of internal organs and functions which, although highly perfected, is essentially molluscan.
Along the Atlantic coast of the United States, particularly in its northern portion, occur several examples of cephalopods belonging to the genera Ommastrephes and Loligo, all the species of which are referred to, in common parlance, as "squids." They frequent shallow water, and are often to be found in weirs, darting about with rapid, spasmodic movements, or perhaps lying motionless on the bottom. The squids enter the weirs for the purpose of capturing the young mackerel which are caught in these traps. The squid lies quietly upon the bottom, which it simulates so closely in color as to be almost invisible, and when [pg465] a school of fishes swims over it, darts suddenly into the midst of it, seizes a fish with its sucker-bearing arms, and kills it by the bite of its parrot-like beak or jaws. Sometimes it happens that squids, while pursuing fish too near shore, precipitate themselves upon the beach, where they flounder about, ejecting water from their siphons, which pushes them only farther away from the water, and squirting out "ink" from their ink-sacs in a vain endeavor to hide themselves from view. On bright moonlight nights squids often go ashore in vast numbers, and perish within a few inches of their native element, which they seem to be unable to regain. These creatures usually swim backward, and the theory is that, dazzled by the bright light of the moon, they continue to gaze at it while swimming, and if there happens to be a shore in the direction of their movements, they suddenly find themselves beached. The fishermen of Canada and New England take advantage of this habit and capture great quantities of squids by placing bright lights in the bows of their boats and then rowing toward shore, thus driving the squids out of the water. The Banks fishermen use them as bait for catching cod. The right claimed by American fishing-schooners to purchase squids in Newfoundland has helped to keep alive the quarrel between Canadian and American fishermen, which has vexed their respective governments for many years.
The range in size among the species of this class is very remarkable. The little sepiolas are about an inch long; the squids of our coasts vary in length from eight inches to one foot; and the giant Architeuthis of the North Atlantic measures, often, fifty feet from the end of its arms to the tip of its tail. Such a creature, with its long arms provided with suckers, its powerful jaws, and its rapid, alert movements, is a formidable foe. These animals have been the basis of many legends about sea-serpents and sea-monsters. A gruesome story of an octopus is told by Victor Hugo in "The Toilers of the Sea," where he gives a thrilling account of a man's encounter with a devil-fish in a cave. One who has read this tale has a vivid picture in his mind of the giant squid, and the danger of meeting one of these many-armed foes. Victor Hugo's devil-fish, [pg466] however, is an animal not true to nature, but a composite, having the attributes of the polyp and of the octopus, and the name of a large ray of Southern waters, a real fish, the Cephaloptera, known in its localities as devil-fish. This monstrosity of the novelist's imagination has, however, done more to acquaint the general public with these interesting cephalopods than have the descriptions of scientists.
The giant squid, which is such a dangerous foe, has its own enemy in the sperm-whale. The cachalot swims through the water with its lower jaw hanging, the cephalopod grasps the jaw, and the whale then shuts his capacious mouth upon it. Whalemen describe conflicts between these enormous creatures, the whale always being the conqueror. Sperm-whales killed by man often eject great quantities of the squids in their death throes, showing this food to be almost their exclusive diet.
The name "cephalopod," meaning "feet around the head," is descriptive in part of their anatomy. The head is usually marked off by a neck-constriction, and it bears two highly organized eyes. The foot is fused in part with the head above the eyes and around the mouth; on the upper side it is divided into eight or ten long arm-like processes, bearing suckers, which act as organs of prehension. The under part of the foot forms a tube called the funnel. Through the funnel the animal expels water from the mantle cavity, and thus propels itself through the water.
The mantle covers the body of the animal, and is a cup-shaped or conical envelope, open only at the anterior end, through which project the head and siphon or funnel. It is attached to the body by a line on the dorsal side, the anterior margin being free and open, but provided with an arrangement of cartilages by which it can be hooked on to the siphon, thus completely closing the entrance to the mantle cavity. The mantle is very muscular, and is constantly expanding and contracting, taking water into the mantle cavity through the mantle opening for respiratory purposes, or expelling it through the siphon for propulsion; in the latter case the mantle opening is closed at the moment of ejecting the water. When the siphon is in its normal position [pg467] the animal swims backward; but it can be turned back over the edge of the mantle, giving a forward movement.
The surface of the mantle is covered with pigment-cells (chromatophores). There are sets of chromatophores containing different colors. The cells are opened or closed at will by muscular action of their walls. When open the cells seem to fuse together, giving a solid color-surface, or spots of color as desired, in blue, red, yellow, or brown; when closed they seem as specks on the almost transparent tissues of the animal. Flashes of changing color follow one another with great rapidity over the living animal. In swimming it assumes the color of its surroundings.
Another curious means of protection possessed by the dibranchiate cephalopods is an ink-bag, the brown or black secretions of which are ejected through the siphon, clouding the water when the animal wishes to escape from danger. The ink taken from the ink-sac of Sepia is an article of commerce.
Only Nautilus and the female Argonauta have the characteristic external shell of mollusks. In all other forms the shell is internal or is invested in the integument of the mantle. Of such is the cuttlebone of commerce, which is a calcareous leaf-like body obtained from Sepia, the cuttlefish. The common squid Loligo has a horny substance situated in the dorsal side of the mantle, called the pen. Spirula has a spiral internal shell divided into chambers. Vast numbers of these shells are cast upon the beaches of the Pacific Islands, and they have also been found on the shore of Nantucket.
Cephalopods are separated into two subclasses. In the first, the Tetrabranchiata, there are four branchiæ, four nephridia, and four auricles. They are without an ink-sac, and have the foot divided into lobes bearing sheathed tentacles. Nautilus is the only genus.
Genus Nautilus
Probably the best-known cephalopod is Nautilus—the pearly-shelled, chambered nautilus made immortal in the beautiful poem of Oliver Wendell Holmes. The shell of Nautilus is a flat spiral; the interior is divided by septa forming a series of chambers; [pg468] the septa are perforated, and through the opening runs a tube or prolongation of the body, the siphuncle, which extends to the tip of the shell. The chambers are filled with gas. The animal in the course of its growth moves forward into a newly formed chamber and builds a new septum, closing the cavity last occupied. Nautilus lives among the coral reefs of the southern Pacific. Its four or five species are the remnants of a once very extensive race of cephalopods. The fossil remains of many species of Nautilus, together with various other genera of shell-bearing cephalopods, indicate that this group has seen its best days. The dibranchiate genera, however, appear to have reached their maximum at the present day.
The second subclass, the Dibranchiata, is characterized by two branchiæ and two auricles. The main part of the foot is divided into eight or ten long arms provided with numerous suckers arranged in from one to four rows on the ventral side of the arms. They have also an ink-sac. The Dibranchiata are divided into two orders: the Octopoda, which have eight arms, and comprise the Octopus and Argonauta; and the Decapoda, which have ten arms, and comprise Spirula, Ommastrephes, Sepia, and Loligo. In these animals two of the ten arms are longer than the others, and these tentacular arms have suckers only on their broadened, club-like ends, they are kept retracted within grooves, one on each side of the head, except when needed for prehension. [pg469]
Genus Argonauta
A. argo. In this species, the paper-nautilus, the shell, which is possessed only by the female, is not chambered. The animal rests in the shell, but has no organic connection with it other than by the membranous expansions at the extremities of the two dorsal arms, which secrete it and hold it in place. The purpose of the shell is to hold and protect the eggs.
Genus Loligo
L. Pealei. In this species the body is cylindrical, tapers to a point, and has a flat appendage in front. The fins are terminal, half, or more than half, as long as the body, united in a point posteriorly, and obtusely rounded on the outer angles. The head is a little narrower than the mantle. There are eight sessile, sub-triangular arms furnished with two series of suckers which are hemispherical and stalked, and two long retractile tentacular arms dilated at the extremities, bearing four rows of suckers. It has a pen- or quill-shaped cartilaginous substance in the dorsal integument of the mantle. Common from Cape Cod to South Carolina.
L. brevis. A small short-bodied species, with short rounded caudal fins. It is common from South Carolina to Florida, and extends as far north as Delaware Bay.
Genus Ommastrephes
O. illecebrosus. A species similar to L. Pealei, but with shorter fins. These are broad and pear-shaped, one third wider than they are long, and usually reach less than one half the length of the mantle. They are straight on the posterior end, and form nearly a right angle, while the upper margins are rounded. The head is as broad as the mantle. The arms are stout and taper to an acute point. The tentacular arms are long when expanded. The species is abundant from Cape Cod to Newfoundland, and is found as far south as Newport, Rhode Island. This is the squid which is used so extensively in the Banks fishery as bait. It [pg470] visits the shores in large schools, presumably in pursuit of prey, and often itself becomes the victim. It is a beautiful creature from its varying colors, which are truly wonderful. It changes in a moment from vivid red to deep blue, purple, orange, and so on, the colors often passing over it in flashes as it swims along. [pg471]
The most conspicuous animals of this class are the ascidians, which are common objects on rocky coasts. The simple ascidians are peculiar leathery, sac-shaped bodies which send out jets of water when touched. This habit gives them the common name of "sea-squirts." Some are highly colored, especially those of Southern waters; others are somber, unattractive bodies, often growing in masses. The compound ascidians are gelatinous colonies, sometimes forming thin incrustations, sometimes jelly-like masses, on seaweeds, shells, etc. This class comprises also the beautiful Salpa, a genus of free-swimming animals having transparent bodies encircled by rings of muscular bands, and in one stage forming chains of attached organisms which swim on the surface of the sea and of bays.
The tunicates are by some authors classed with the vertebrate animals because in the larval stage they have a notochord; this disappears, however, in the adult form, and the animals are considered degenerates. The tunicates are interesting to biologists from the remarkable changes they undergo in their life-history, and (in some genera) the marked phase of alternation of generation. One of their curious anatomical features is the blood-vascular system. The circulation is propelled by wave-like contractions of the heart, which, after forcing the blood one way for a time, stops and reverses the blood-current. The tunicates are widely distributed, and occur at all depths. (Plate LXXXV.) [pg475]
Genus Salpa