Most of the Echinoderms have the power of regenerating lost parts. That is, if a starfish loses an arm (ray) through accident, a new ray will grow out to replace the old. And this power of regeneration extends so far in the case of some starfishes that if very badly mutilated they can practically regenerate the whole body. This amounts to a kind of asexual reproduction. Some species, too, have the peculiar habit of self-mutilation. "Many brittle stars and some starfishes when removed from the water, or when molested in any way, break off portions of their arms piece by piece, until, it may be, the whole of them are thrown off to the very bases, leaving the central disc entirely bereft of arms. A central disc thus partly or completely deprived of its arms is capable in many cases of developing a new set; and a separated arm is capable in many cases of developing a new disc and a completed series of arms." In some of the sea-cucumbers "it is the internal organs, or rather portions of them, that are capable of being thrown off and replaced, the œsophagus ... or the entire alimentary canal, being ejected from the body by strong contractions of the muscular fibres of the body-wall, and in some cases, at least, afterwards becoming completely renewed." =Classification.=--The Echinodermata are divided into five classes, viz., the Asteroidea or starfishes, "free Echinoderms with star-shaped or pentagonal body, in which a central disc and usually five arms are more or less readily distinguishable, the arms being hollow and each containing a prolongation of the body-cavity and contained organs"; the Ophiuroidea, or brittle-stars, "star-shaped free Echinoderms, with a central disc and five arms, which are more sharply marked off from the disc than in the Asteroidea and which contain no spacious prolongations of the body-cavity"; the Echinoidea, or sea-urchins, "free Echinoderms with globular, heart-shaped, or disc-shaped body enclosed in a shell or corona of close-fitting, firmly united calcareous plates"; the Holothuroidea, or sea-cucumbers, "free Echinoderms with elongated cylindrical or five-sided body, ... with a circlet of large oral tentacles"; and the Crinoidea, or feather-stars, "temporarily or permanently stalked Echinoderms with star-shaped body, consisting of a central disc, and a series of five bifurcate or more completely branched arms, bordered with pinnules." =Starfishes (Asteroidea).=--The starfishes feed on other marine animals, especially shell-fish and crabs. They are also reputed to destroy young fish. By means of their sucking-tubes, or tube-feet with sucker tips, they can seize and hold their prey firmly. They do much injury to oyster-beds by attacking and devouring the oysters. When attacking prey too large to be taken into the mouth the starfish everts its stomach over the prey and devours it. The stomach is afterward drawn back into the body-cavity by special muscles.