The mouth of the sea cucumber is surrounded by plumed tentacles which can be retracted at will, and which are used in capturing the smaller living things that form its food. Like the earthworm, it will often swallow large quantities of sand, from which it digests the organic matter contained. The body-wall of the _Holothuroidea_ is strong and muscular, and is strengthened by the presence of numerous spicules of carbonate of lime, often in the form of little anchors, wheels, and crosses, while the outer surface is rough and slimy, and often of a colour so closely resembling the surroundings of these animals that they are not easily observed. This feature is one of great value to the creatures, since they have no means of defence from their enemies, and seem to owe their safety entirely to their protective colouring. [Illustration: FIG. 115.--THE SEA CUCUMBER] There are several species of sea cucumbers on our coasts, but all inhabit deep water and are seldom to be seen above low-water level. They are, as a rule, easily obtained from fishermen, who will bring them in when requested to do so. Live specimens may be kept for a considerable time in the indoor aquarium, and seem to prefer a rocky bottom on which they can hide under stones at times, and a bed of sand on which they will occasionally crawl. They will readily devour small molluscs and crustaceans, and will partake of dead organic matter in a partially decomposed state. The following tabular summary of the classification of Echinoderms may possibly be of use for reference:--