the power of locomotion. In some of the sea-cucumbers five equidistant rows of tube-feet extend from one end of the body to the other, and the animal crawls worm-like upon any side that happens to be down; but certain spe

cies living in the sand, where tube - feet will not work satisfactorily, have lost all traces of them, and creep like an earthworm from place to place. In all the sea-cucumbers the feet, situated near the mouth, have been curiously modified to form a circlet of tentacles, which range in form from highly branched to short and thick structures, and in function from respiratory organs and those of touch to contrivances for scooping up sand and conveying it to the mouth.

147. Food and digestive system. In the echinoderms the body-wall is comparatively thin (Fig. 98), and encloses a great space, the body-cavity, in which the digestive and reproductive organs are contained. As the former in various species is adapted for acting upon very different kinds of food, it shows many modifications; but there are a few principal types which may be briefly considered.

In the starfishes the mouth enters almost directly into the cardiac division of the stomach, a capacious, thin-walled sac, much folded and packed away in the disk and bases of the arms (Fig. 98, b). This in turn leads into the second pyloric portion (a), with thicker walls and dorsal, to the first, from which a short intestine leads to the exterior, near the center of the disk. Another conspicuous and important feature is the so-called liver, consisting of a pair

of closely branched, fluffy glands (1), extending the entire length of each arm and opening into the pyloric stomach.

The starfishes are carnivorous and highly voracious, devouring large numbers of barnacles and mollusks which happen in their path. If these are small and free they are taken directly into the stomach, but when one of relatively large size is encountered the starfish settles down upon it, and, slowly pushing the cardiac stomach through the mouth, envelops it in the folds. Digestive fluids are now poured over it, and the victim is speedily despatched and in a partly digested condition is gradually absorbed into the body, leav

FIG. 98. Dissection of starfish to show: a, pyloric stomach; b, bile-ducts (above), cardiac stomach (below); b.c., body-cavity; f, feet; g, spines; i, intestine; 7, liver; m, mouth; m.p., madreporic plate; r, reservoir; r.c., ring canal; r.m., stomach retractor muscle; r.v., radial vessel; s, stone canal; t, respiratory tree.

ing the shell and other indigestible matters upon the exterior. Oysters and clams close their shells when thus attacked, but a steady, continuous pull on the part of the starfish finally opens them, and the stomach is spread over the fleshy portions with speedily fatal results. In the interior of the body the food is transferred to the pyloric stomach, subjected to the action of the liver, and when completely dissolved is borne to all parts of the body.

The digestive system of the starfishes, with its various subdivisions and appendages, is in some respects more complicated than in the other classes. This is most strikingly the case with the serpent-stars, where the entire system for disposing of the minute animals and plants on which it feeds consists of a simple sac communicating with the exterior by a single opening-the mouth.

In the sea-cucumbers large quantities of sand are taken into the body, and the minute organisms and organic matter are digested from it. In the sea-urchins the mouth is provided with five teeth, and the food consists of minute bits of seaweeds, which these snip off. Such diets evidently require a comparatively simple digestive apparatus, for in both it consists throughout its whole extent of a tube of equal caliber, in which the various divisions of esophagus, stomach, and intestine are little, if at all, defined. This is usually somewhat longer than the body, and therefore thrown into several loops; and in the sea-cucumbers its last division is expanded and furnished with more highly muscular walls, which aid in respiration.

148. Development.-With but a few exceptions, the eggs of the echinoderms are laid directly in the surrounding water, and for many days the exceedingly minute young are borne great distances in the tidal currents. During this period they show no resemblance to their parents, and only after undergoing remarkable transformations do they assume their permanent features. In every case they have a five-rayed form in early youth, but in several species of starfishes additional arms develop until there may be as many as twenty or thirty.

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CHAPTER XIII

THE CHORDATES

149. General characters.-Up to the present time we have been studying the representatives of a vast assemblage of animals whose skeletons, if they have any at all, are located on the outside of the body. In the corals, the mighty company of arthropods, and the echinoderms, it is external. On the other hand, we shall find that the animals we are now about to consider, the fishes, frogs, lizards, birds, and mammals, are in possession of an internal skeleton. In some of the simpler fishes and in a number of more lowly forms (Fig. 99) it is exceedingly simple, and consists merely of a gristlelike rod, the notochord (Fig. 101, nc), extending the length of the body and serving to support the nervous system, which is always dorsal. This is also the type of skeleton found in the young of the remaining higher animals, but as they grow older the notochord gives way to a more highly developed cartilaginous or bony, jointed skeleton, the vertebral column.

In the young of all these back-boned or chordate animals, the sides of the throat are invariably perforated to form a number of gill-slits. In the lower forms these persist and serve as respiratory organs, but in the higher animals they disappear in the adult. The chordates are thus seen to be distinguished by the possession of a dorsal nervous cord supported by an internal skeleton and by the presence of gill-slits, characters which separate them widely from all invertebrates.

The chordates may be divided into ten classes, seven of

which (the lancelets, lampreys, fishes, amphibians, reptiles, birds, and mammals) are true vertebrates, while the others embrace several peculiar animals of much simpler organization.

150. The ascidians.-Among the latter are a number of remarkable species belonging to the class of ascidians or

sea-squirts (Fig. 99). These are abundantly represented along our coasts, and are readily distinguished by their sac-like bodies, which are often attached at one end to shells or rocks. On the opposite extremity two openings exist, through which a constant stream of water passes, bearing minute organisms serving as food. When disturbed they frequently expel the water from these pores with considerable force, whence the name

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sea-squirt." While many lead solitary lives, species are often closely

numerous individuals of other packed together in a jelly-like pad attached to the rocks, and others not distantly related are fitted to float on the surface of the sea.

The young when hatched resemble small tadpoles both in their shape and in the arrangement of some of the more important systems of organs. For a few hours each swims about, then selecting a suitable spot settles down and adheres for life. From this point on degeneration ensues.