Sea Cucumbers are sort of second cousins to the sea urchins and rather more distantly related to the starfish. As the name indicates, they are shaped like a cucumber, and hundreds of little feet on the side heighten the resemblance, as they recall the spines on the vegetable. Inside they have a coiled intestine, usually filled with mud and the contained vegetable and other debris. With us no use is made of the animals, but they are taken in great numbers in the South Seas, and sent to China, where, as trepang, they form an ingredient in soups.
Sea-lilies are deep-sea animals, once numerous and flourishing, but now comparatively rare, and only to be obtained by dredging in the deeper parts of the ocean. They are fixed by a long, jointed stalk bearing circlets of sensitive threads and terminating in a cup, in the center of which the mouth is situated. Radiating from the edges of the cup are five branching, feather-like arms, all of which are grooved above, the grooves uniting, and finally converging to the mouth. They are beset with cilia, and minute organisms are conducted inward along them to serve as food.
Sea Urchins are radiated animals which are usually shaped like a flattened sphere. They have a mouth, surrounded by five chisel-shaped jaws at one pole, while the whole outer surface is covered with slender, movable spines. Between the spines are numbers of slender, flexible, tubular feet, which pull the body along, while the spines act more like true feet. The animals feed mostly on seaweeds. They have no economic value with us, but in Europe the eggs of some species are eaten, forming part of the frutti di mare of every Italian seaport.
Starfishes.—Starfishes are among the most familiar objects of the seashore, and the commonest kinds, such as the five-finger (Asterias rubens) and the comb-star (Astropecten aurantiacus) possess five radiating arms. The mouth is in the center of the under side, and leads into a capacious stomach, of which the first part can be protruded from the body to surround such prey as mussels and oysters.
A starfish crawls slowly by means of numerous tube-feet, which are lodged in five grooves radiating from the mouth, and make up a part of the water-vascular system, so called because it is full of sea-water. At the end of each arm is an unpaired tube-foot acting as a feeler, while on its under side there is an orange-red eye-spot.
The water-vascular system assists in breathing. It was probably first evolved in the interests of respiration, and this is its chief use in the sea-lily. Some of the spines are formed of little, two-bladed pincers, which clean the surface of the body.
Starfishes are remarkable for their powers of restoring lost parts. A detached arm can grow a fresh disc and another four arms.
ANIMALS THAT APPROACH THE SIMPLEST FORMS OF LIFE
WORMS, LEECHES, SEA-ANEMONES, CORAL-POLYPS, JELLY-FISHES, SPONGES
Several groups of the lower animals are collectively known as Worms, though most of these groups are but remotely related. Ringed worms are elongated creatures in which the body is made up of a considerable number of rings or segments, most of which are, on the whole, much alike. There is often a well-marked head, but no distinct thorax and abdomen, as in an insect or crayfish. Two subdivisions are recognized: (a) Bristle-worms (Chætopoda) and (b) Leeches (Discophora).
Bristle-worms include a host of marine worms, together with some that live in fresh water, and also the earthworms. Their average characters are best understood by examining one of the commonest shore-worms, known as the sea-centipede (Nereis). Here the segments are very clearly seen, and almost every one of them bears a pair of unjointed conical foot-stumps, used for crawling.