PHYLUM ECHINODERMATA 201 



232. Specializations. — Specialization or adaptation is the develop- 

 ment of structures which fit an animal to perform certain particular 

 functions or to meet certain peculiar conditions in the environment. 

 The echinoderms show some of the most marked examples of specializa- 

 tion to be found anywhere in the animal kingdom. Among these are (1) 

 the entire water-vascular system, (2) the spines and plates which form 

 the exoskeleton, (3) the pedicellariae, (4) the dermobranchiae, and 

 (5) the amebocytes. 



233. Classification. — The phyhim Echinodermata (e ki no der' ma ta; 

 G., echinos, hedgehog, and dennatos, pertaining to skin) is divided into 

 five classes: 



1. Asteroidea (as ter oi' de a; G., aster, star, and eidos, form). — The 

 starfishes. 



2. Ophiuroidea (o fi u roi' de a; G., ophis, serpent, oura, tail, and 

 eidos, form).- — The brittle stars and serpent stars. 



3. Echinoidea (ek i noi' de a; G., echinos, hedgehog, and eidos, foTin). — 

 The sea urchins and sand dollars. 



4. Holothurioidea (hoi o thti ri oi' de a; G., holothourion, water polyp, 

 and eidos, form).^ — The sea cucumbers. 



5. Crinoidea (krl noi' de a; G., krijion, lily, and eidos, form). — The 

 feather stars and sea lilies. 



234. Asteroidea. — The general characteristics of this class are illus- 

 trated by the starfish. The bases of the rays take up the entire circumfer- 

 ence of the disc and thus are not definitely marked off from it. The 

 number of rays varies in different species from 5, which is the most usual 

 number, to more than 40. Though usually an odd number, it is not 

 invariably such, since there are forms in which the number is regularly 

 six. In some the disc is small and the rays are long and slender; in 

 others the disc is large and the rays short and broad. This shortening 

 and broadening of the rays may go so far as to produce pentagonal types. 

 Starfishes are rather generally distributed, being absent only from the 

 polar regions. 



235. Ophiuroidea. — This class differs from the preceding in that it 

 possesses slender rays sharply marked off from the disc and in that the 

 rays have no ambulacral grooves. Owing to the slenderness of the rays 

 none of the viscera extends into them and they are exceedingly flexible 

 and capable of very rapid movement. A madreporite is found on the 

 ventral surface, but tube feet, if found at all, exist only on the ventral 

 surface of the disc, adjacent to the mouth, where they serve as tactile 

 organs and pass the food into the mouth opening (Fig. 108). The types 

 known as brittle stars and serpent stars are found under stones on the 

 beach at low tide. When the tide is in they wander more or less about 

 the bottom, having somewhat the same feeding habits as the starfishes, 

 but cannot eat objects of any considerable size. Brittle stars are capable 



