204 



METAZOAN PHYLA 



234. 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. 





Fig. 116. — The 20-rayed sunflower star, Pycnopodia helianthoides (Brandt). Twenty 

 to twenty-four rays are produced, usually in even numbers, although an occasional specimen 

 is found with an odd number. Very young individuals may have as few as six rays. This 

 species is one of the largest of starfishes, often attaining a diameter of 2 or more feet, and is 

 common from California to Alaska. {Photograph by 0. Wade.) 



235. Classification. — The phylum Echinodermata (e kl no der' ma ta; 

 G., echinos, hedgehog, and dermatos, 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 (6 fi u roi' de a; G., ophis, serpent, ou7-a, tail, and 

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



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

 The sea urchins and sand dollars. 



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

 and eidos, form). — The sea cucumbers. 



