The crinoids are inhabitants of deep water, where they grow in great numbers, forming beds of sea-lilies. Their general form, which suggests the lily, and their feather-like manner of branching, give them the two common names of sea-lilies and sea-feathers. They have a long, jointed stalk, one end of which is attached, while the other bears the disk of the animal. From the disk emanate five arms, which divide near the base, making [pg235] ten arms in all. The arms have short branches along the sides. Ambulacral grooves follow the center of all the arms and branches. The mouth is in the center of the upper side, which in this case is the ventral surface. The genus Pentacrinus remains permanently attached, but Comatula, at a certain stage of development, separates from the stalk and swims freely about by means of its arms. It can attach itself temporarily by tubular processes, which are developed on the dorsal surface at the point where the stem was attached. The ossicles, or plates which cover the dorsal surface, are free, making the crinoid an animal of innumerable joints.
The crinoids are particularly interesting from the fact that they have existed from early geologic times, and their history is written in stone. In the early ages they were the only class of echinoderms, and their evolution into other forms can be traced through successive geologic periods. They existed in such vast numbers that the fossil forms are plentiful and are familiar to every student of geology, and are known as stone-lilies and encrinites. To-day they are decadent, there now remaining only twelve of the two hundred genera which existed formerly. A fine bed of crinoids is found off Cuba, on the slope of the coast where the water rapidly deepens from one hundred to two hundred fathoms. [pg236]
Although the crinoids are deep-water forms, and are never found on the beach, they are given here because they follow in the system of classification and illustrate another curious class of echinoderms. The brief description given may add interest to the fossil forms to be seen in museums.
Abdomen: The posterior part of the body.
Antennæ: Articulated appendages which immediately precede the mouth.
Anten´nules: The anterior of the two pairs of feelers of the head.
Bira´mous: Having two branches.
Carapace: A thin chitinous shell covering the cephalothorax.
Cephalic: Pertaining to the head.
Cephalotho´rax: The united head and thorax.
Chela: The pair of pincers, or claw, which terminates some of the appendages.
Che´liped: A leg with chela, or claw, at the end.
Endop´odite: The inner one of the two main divisions of the limb.
Epime´ra: Divisions on the ends of the segments of amphipods and isopods, and belonging to the legs.
Epip´odite: A third branch of a limb.
Epis´toma: A triangular area in front of the mouth.
Exop´odite: The outer one of the two main branches into which the typical limb of any segment is divided.
Gastric mill: The first half of the stomach, where food is ground.
Mandibles: The pair of appendages next behind the antennæ.
Maxil´læ: The first pair of appendages after the mandibles.
Maxillipeds: The three pairs of appendages after the maxillæ.
Metame´res: Segments.
Orbits: Eye-cavities, peculiar to the higher Crustacea.
Prosto´mium: The region in front of the mouth; the preoral part of the head.
Rostrum: The beak in front of the head.
Segment: A part cut off or marked as separate from others.
Somite: A segment of the body.
Squame: A scale.
Telson: The last segment of the abdomen.
Thorax: The middle part of the body.
This subdivision of the animal kingdom includes insects, centipedes, spiders, and crustaceans, which together constitute more than half the known species of animals. Although these animals are so unlike in general appearance, it is easy to recognize the common characteristics which place them together in one group. The name Arthropoda, meaning "jointed-footed," suggests perhaps one of the most obvious points of resemblance. The Arthropoda have bilateral symmetry, one side of the body being like the other; they are covered with a horn-like material (chitin); they are divided into segments; the segments have appendages; and the appendages are jointed so as to admit freedom of motion. Their manner of growth is peculiar; they cast off their rigid external coverings and secrete larger coats of mail, and at these periods increase in size or undergo metamorphosis.
There are such modifications of these general features as the habits of the different species demand. For instance, the appendages may be constructed for walking, swimming, boring, sucking, or the seizure and preparation of food. In some animals the appendages form a part of the breathing-organs, in others are used as organs of sense. Every detail of the organism, down to the hairs, has its special use and function.
The crustaceans vary in size from microscopic minuteness to two feet or more in length. The giant crab of Japan (Kaempferia kaempferi) exceeds this, being commonly from eight to twelve feet [pg245] across the arms, and is said to reach even greater proportions. Crustaceans live on land and sea, and in both fresh and salt water; they may be parasitic, sedentary, or free and active. There are said to be over ten thousand species, which include crabs, lobsters, shrimps, beach-fleas, wood-lice, barnacles, and water-fleas. Nine tenths of the species are marine; of these some are pelagic, and their transparent forms constitute a part of the plankton. Others live on the bottom in deep water and attain a large size. A vast number live in the littoral zone and form a considerable part of that crowded community. They are scavengers and freebooters, being great fighters, as well as predatory in their habits. As a rule, they feed on dead organic matter, which they consume in surprising quantities; some have also a vegetarian diet. They have, like starfishes, the singular power of throwing off their limbs and growing new ones.
The Crustacea derive their name from the nature of their crust or covering. This is earthy and brittle, not stony, like the shells of mollusks. This crust, or exoskeleton, has various degrees of firmness, from that of a delicate polished cuticle, seen in small forms, to the heavy armor of some crabs and the shell-like character of the covering of barnacles.
The lobster and crab are perhaps the most comprehensive examples as well as the largest in size of the class, the vast majority of the species being minute forms. The ocean swarms with varieties too small to be noticeable to the naked eye.
It is the province of Crustacea to consume decaying organic matter, both animal and vegetable, and in doing this they perform a great service in purifying the waters of the pool, the shore, and even the sea. Not only in this are they serviceable, but to a great extent they serve as food for other animals, their flesh being palatable and preferred to that of other animals whose diet would seem to make them more wholesome. The small forms exist in such myriads, and increase so rapidly, that, although extremely small, they furnish an inexhaustible supply of food. Even some whales subsist upon these minute animals, and for man the larger species are articles of diet that are counted among the luxuries. The crustaceans are remarkable for their varied forms and for their [pg246] perfect adaptation to many different conditions of life. There is no class in the animal kingdom which presents so wide a range of organization, or whose structure deviates so widely from the type form. These differences lie chiefly in the external characters and in the structure of the appendages.
Some species are solitary, like lobsters and crabs; others, like shrimps, are gregarious and live in immense shoals. In the habits of these animals there is much which excites wonder and interest.
The body of a crustacean is divided into segments, which are sometimes distinctly separated, like joints, and sometimes fused into one piece. The head part is called the cephalic portion, the middle section is the thorax, and the posterior part is the abdomen. The horn-like covering of the thorax is the carapace; where it projects over the head it is the rostrum. When the head and thorax are united, as in most Crustacea, the anterior or front portion of the animal is called the cephalothorax. Each segment has a pair of appendages. Each appendage has a joint attached to the body; from this arise two jointed branches, the endopodite and the exopodite, the inner and the outer foot. The appendages are modified to perform special functions. In front of the mouth are two pairs: the smaller ones are the antennules and have ears at the base; the larger ones are antennæ and are feelers. At the base of the antennæ are the green glands, said to have renal functions. Both the antennules and the antennæ are fringed with hairs, which aid in the sense of touch and perhaps of smell. On each side of the mouth are the mandibles. Then come the maxillæ and the maxillipeds, used in capturing and tearing the food and conveying it to the mouth. Thus it will be seen that some of the appendages around the head are connected with the senses, and others are used in eating. Those of the thorax are for walking; those of the abdomen for swimming, guarding the eggs, etc. The food taken into the mouth passes into the stomach through a short passage. The stomach is divided into two parts. The front [pg247] one contains three long teeth which meet laterally and grind the food; this is known as the gastric mill.
When sufficiently fine, the food passes through a strainer of stiff bristles into the smaller portion of the stomach, where it is partially digested, and from there enters a long, straight intestine which reaches the length of the body and opens to the outside on the under side of the telson, or last segment. A large liver also pours its secretions into the intestine. The green substance commonly called "fat" in lobsters is the liver.
The heart consists of an elongated tube, or a short sac, which lies directly under the integument of the back. From this heart-sac, blood, which is colorless, is sent by arteries to all parts of the body; it then collects in spaces called venous sinuses, from which it goes to the gills, and thence back to the heart.
The nervous system begins in a large ganglion in front of the mouth, called the brain; from this two branches arise, which pass on each side of the digestive organs, meeting in ganglia in each segment and extending the whole length of the body. The gills, by which the animal breathes, are upon the limbs, or on the walls of the body immediately adjacent to them, and are generally inclosed in special chambers. In lobsters and crabs two such chambers are found under the flaps of the carapace, above the walking-legs. Gills are divided so as to present much surface to the water, from which they absorb oxygen. They are like a dense mass of little tubes arranged along a central tube. The class has two kinds of eyes, simple and compound; the latter are composed of a number of eyes. In some species the eyes are placed on the ends of movable stalks, which enable the creature to see in all directions and from a higher plane than the body occupies. As a rule, the eyes occur in the head region, but in the shrimp Euphausia they are on the thorax and abdomen. In barnacles simple eyes exist in the young stage, but in adult forms there are no apparent visual organs. The ear (so called) consists of a sac containing small silicious particles suspended in fluid. Numerous fine hairs on the inner surface of the sac connect with nerve-fibrils.
The organs of hearing are in various places. In decapods, or the larger Crustacea, they are at the base of the antennules. [pg248]
The muscles are white bundles of fibers, and are in strips, which is an indication of power and activity. There are four important sets: twisted ventral muscles which bend the tail and are particularly large and strong, those which straighten the tail, those moving the appendages, and those which work the gastric mill.
The eggs, after being discharged, are attached to the abdominal legs of the mother by a kind of cement, or they are carried in pouches attached to the thorax. In these positions they mature, and hatch at different stages of development in different species. Some emerge with three pairs of legs, and are known as Nauplius; some are Zoëa, having a carapace and abdominal segments, but no abdominal appendages. Another stage is Megalops, with large stalked eyes. Others are hatched as miniature adults. These names were given when the embryo stage was not recognized and the larvæ were thought to be distinct species.
The growth of the animal is effected by moulting. The Crustacea are named from the crust-like covering which envelops them. It is a horny material, called chitin, in which are deposited particles of carbonate of lime, making a rigid envelop which would prevent all freedom of motion, were it not that there are spaces free from lime, and thus flexible joints are left. When the animal expands it throws off this hard covering and secretes a new and larger one. As the time of moulting approaches, the old covering becomes loosened, and a delicate new one is formed beneath it. The old shell splits open across the back just behind the carapace, and the soft animal withdraws first its cephalothorax and then its abdomen, leaving the cover complete, including even the covering of the eyes and the lining of the stomach. The [pg249] new shell is rapidly hardened, being already formed when the old one is cast, and the animal regains its normal condition in about a week; in the meantime it is defenseless, and lies quiet in some secluded place. Moulting is an exhausting process, and is attended with great dangers. A great mortality occurs at this time from accidents, from weakness, and also from helplessness in case of attack.
The hair-like processes scattered over the shell, often like fringes, are said to be organs of feeling.
These are Crustacea of small, often microscopic size, of comparatively simple organization, and with appendages adapted to serve the purpose of respiration. These minute animals may be obtained by skimming the surface of the water with a muslin net, preferably at night, then washing off the inside of the net with a small quantity of water into a glass dish. Place the dish on a dark surface before a light, and the little creatures will gather toward the light, and may be satisfactorily observed with a glass.
ORDER COPEPODA
This order, though composed of minute forms, is one of great economic importance, from the fact that the little crustaceans exist in vast numbers and furnish a very considerable part of the food of many fishes. Cyclops is the most common of the fresh-water, and Cetochilus of the marine genera. These, together with other genera, swarm in water wherever life exists, from the smallest pools and ditches to the broad surface of the ocean. Without having drawn a surface-net on some sheltered bay, it is difficult to have an idea of the myriads of Entomostraca in the sea. Although nearly transparent and of such delicate texture as to be almost jelly-like, they sometimes color the sea with a reddish tint for miles. Whales which have baleen, or fringes of whalebone, in the mouth subsist on these small organisms, which are called "brit" by the whalemen. The whales, sometimes in schools, rush through the water with open mouths, engulfing these little [pg250] crustaceans, the baleen straining them from the water. Although devoured in such immense quantities, and sometimes lying dead in sheets of scum on the surface of the water, they maintain their numbers by the exceeding rapidity with which they reproduce. It has been computed that the descendants of one Cyclops may number in one year 4,500,000,000, provided all the young reach maturity and produce a full number of offspring.
One of the free marine forms, Sapphirina, is of especial interest, as it surpasses all animals in phosphorescence and sparkles by day as well as by night. It is one quarter of an inch long, and is broad and flat.
Besides the myriads of free-swimming copepods, there are parasitic forms in great number. The marine parasitic forms are commonly known as fish-lice. They have various habits, some living as commensals, others attaching themselves to animals only to be carried about; the true parasites live upon the blood and tissues of their hosts, and may fasten themselves to the external parts of the body or to the internal organs. Whales, fishes of all kinds, mollusks, starfishes, jellyfishes, and corals, all have some form of parasite, and many have several different kinds of guests. It is said that the haddock has more than a dozen which infest its external and internal membranes.
Nicothoë is found on the gills of lobsters. The truly parasitic forms are usually very degenerate and lose the characteristics of their order.
ORDER CIRRIPEDIA
("Curled feet")
THE BARNACLES
Barnacles of the genus Balanus (acorn-shells) (Plate LIX.) are familiar objects on rocky shores, which they often whiten with their shells, and those of the genus Lepas are also widely known. The name of the order is descriptive of their curled appendages. The appendages are fringed like feathers and are drawn into or protruded from the shell at will. When extended they are constantly in motion, and create currents which carry food to the [pg251] mouth of the animal, which is dependent upon such food as comes within range of its tentacles.
Their life-history is interesting. The young barnacle, called a nauplius, in no way resembles the adult. When it emerges from the egg it is a free and independent animal, with one eye, three pairs of legs, and a single shell. It swims about for a while and moults several times. It then has two eyes, two shells, and six pairs of legs. At this period it seeks a permanent home, and attaches its anterior end to the object it selects by means of its antennæ, which have become suckers. It makes its hold secure by secreting a cement which permanently fastens it to the spot. It then undergoes metamorphosis, loses its bivalve shell and its eyes, and attains its characteristic cirripeds, or curled feathery legs, and a new shell covering. During these transformations, from the time it becomes fixed until it attains its adult form, the barnacle fasts, living by the absorption of its own animal fat. Its food subsequently consists of the minute animal forms which abound in the sea. Its further growth is by moulting, but parts only of its covering are disengaged; the shell is permanent, and its successive stages of growth are marked upon it by lines, as in mollusks. The lining of the shell, or enveloping skin of the animal, and also the cuticle of the legs are [pg252] shed, and in the spring of the year these thin, glossy casts are found in abundance floating on the surface of the water near the shore.
There are but three orders of barnacles, namely, those in which the shell is directly attached to the rocks, those which are attached to floating objects by a long stalk, and those which are parasitic on animals. A species of the third order infests the whale.
The older zoölogists classed barnacles with the Mollusca, but in 1829 Vaughan Thompson, in the study of their embryology, found that they should be classed with crustaceans, in company with crabs, shrimps, and water-fleas, with which their immature forms show direct relationship.
Genus Lepas
This genus is commonly known as the ship-barnacle, also as the goose-barnacle. It attaches itself to floating logs as well as to ships, but the latter form its principal home; consequently it is a great voyager, and, though common everywhere, is everywhere considered a stranger. The same species are found on ships coming from the most remote and widely separated regions, and so they cannot be considered native to any one locality. They are wanderers on the deep, and grow in such numbers on the bottoms of ships, especially of those which sail in warm seas, that they seriously impede the progress of the vessels. Aside from diminishing its speed, they do a ship no injury.
There was a tradition, which lasted several centuries, that geese were hatched from these shells, which somewhat resemble eggs. Gerard, in the appendix to his "Herball or Generale Historie of Plants" (1597), gives a picture of shells of Lepas growing on a tree, with geese falling from them and swimming about in the water below. His description is as follows: "There are founde in the North parts of Scotland and the islands adjacent called Orchades certaine trees whereon do growe certaine shell fishes of a white color, tending to russet, wherein are conteined little living creatures; which shells in time of maturitie do open, and out of them grow those little living foules whom we call barnakles, in the North of England brant geise, and in Lancashire tree geise; but [pg253] the other that do fall upon the land do perish and come to nothing." He then describes in detail the various transformations, and ends with: "But what our eies have seen and hands have touched we shall declare."
The long, flexible stalk of Lepas is its anterior end. Generally this stalk is only half an inch long, but in some species it attains the length of a foot.
Huxley describes the barnacle as a crustacean fixed by its head and kicking food into its mouth with its legs. The mouth has a pair of small mandibles and two pairs of maxillæ, the last pair uniting to form a lower lip. The thorax has six pairs of branched appendages. The body is enveloped in a fold of skin, to which are attached five shell-like plates. One of these plates is long and narrow, and extends along the dorsal side; two are large and triangular (the terga); two are small and triangular (the scuta), the long point extending downward. These shells are on the free or posterior end.
Barnacles have a nervous system, consisting of a brain and a chain of five or more ganglia, but no special respiratory or circulatory organs are known; the cirripeds, or feet, are supposed to perform these functions. They have also a food-canal, a digestive gland, and excretory tubes. The eggs are carried under the external fold of the skin in flat cakes.
L. anatifera. The shell is bluish-white, showing lines of growth and faint radiating lines emanating from the anterior basal angle. The upper valves are narrow; the long tips point downward, and the top is blunted, leaving a space which is occupied only by a membrane. Near the apex of the shell, at the back, is a distinct angle. The dorsal valve is broad, not much compressed, and is sometimes grooved lengthwise. The cartilage of the shell and the stalk adjoining the shell are orange-colored. The stalk is grayish-brown and the cirri flesh-colored. The stalk is from one inch to six inches long. The shell is one inch long.
L. striata. Shells bluish-white; valves sharply triangular; dorsal valve compressed, forming a ridge; lines radiate from the basal angle of the lower valves and from the upper angle of the terminal valves, starting from the extreme end; the margins have a narrow edge of yellow cartilage; the stalk and cirri are of a dark slate-color; shell and stalk are each about an inch long.
L. pectinata. Shell shorter and less compressed than in the preceding species; lines of growth and radiating lines distinct; a decided line [pg254] runs from anterior base to summit, a little back of the margin; terminal end broadly obtuse (truncated); dorsal valve much compressed, forming a sharp edge serrated with ten to twelve teeth and distinctly striated or furrowed.
Genus Balanus
Balanus is known as the "acorn-shell," or "sea-acorn," and is found in vast numbers and of all sizes between tide-marks, incrusting rocks and the piles of piers. It also may be found attached to floating objects or to shells, or even to living animals, but its usual habitat is stationary. Unlike Lepas, it has no stalk, the shell being directly attached to some object. The body is surrounded by a fold of skin, to which are attached a shell consisting of six or more plates and a fourfold lid, or operculum, consisting of two scuta and two terga. The operculum may be called the door, as the animal opens and shuts it at will and has complete protection when it is closed. If one taps a rock [pg255] incrusted with barnacles, and holds the ear near, the closing of the many doors may be distinctly heard.
The development of Balanus from the larval stage, as also the anatomy of the adult, is similar to that of Lepas. When covered with water and unmolested, there may be seen over a bed of barnacles thousands of tiny fringed feet waving to and fro. The motions look like gestures; they are perfectly regular and rapid, numbering eighty to a hundred a minute. The shell covering formed by barnacles on piles of wharves and bridges is said to be a protective agent; otherwise the barnacles seem to have no economic value in nature, as, unlike other animals, they do not serve, except in very small measure, as food to other classes. The tautog and perhaps some other fishes feed partly upon them. They are sometimes an obstacle to oyster-culture, as they fasten upon the objects intended for oyster embryos, and, growing faster than the latter, soon crowd them off. One species, Coronula diadema, fastens to the skin of whales. It attains the size of two inches in diameter. The shell is half an inch thick and full of cavities, into which the skin of the whale is drawn, giving the barnacle a secure hold. (Plate LIX.)