544 The Animal Kingdom 



With very few exceptions, the echinoderms have separate sexes. 

 There are no accessory ducts, glands, or receptacles for storage of eggs 

 and sperm. The gonads develop from the coelomic walls, and the 

 eggs and sperm are shed directly into the sea water. Almost all have a 

 bilaterally symmetrical free-swimming larva, the dipleurula (Fig. 181). 

 The detailed structure of the larva differs in the various classes. 



The powers of regeneration of most members of this phylum are 

 quite astonishing. In fact, this ability may at times be considered a 

 type of asexual reproduction. A starfish cut in two will regenerate 

 two new starfishes, and at times even a single arm may regenerate a 

 complete new animal. ^^, 



The Classes of the Echinodermata. — Among living echinoderms, 

 there are five recognized classes. 



Class I. Asteroidea. Body usually in five parts, arms flexible, numbering 

 from 5 to 50, not sharply separated from the central disc, with pedicellerae. Ex- 

 ample: starfish, Asterias. 



Class II. Ophiuroidea. Body usually in five parts, arms numbering five, 

 sharply separated from the central disc, flexible. No pedicellariae. Example: 

 brittle star, Ophiura; basket star, Gorgonocephalus. 



Class III. Echinoidea. Body disc-shaped, oval, or hemispherical, without 

 arms. Body skeletal plates forming a test with large moveable spines and pedi- 

 cellariae. Examples: sea urchin, Arhacia; sand dollar, Echinarachnius ; heart 

 urchin, Lovenia. 



Class IV. Holothurioidea. Body elongated, wall leathery or thin, without 

 pedicellariae, skeleton only of scattered plates, cloaca with respiratory tree. Ex- 

 ample : sea cucumbers, Holothuria, Cucumaria. 



Class V. Crinoidea. Stalked forms, with branched arms, without spines, 

 pedicellariae, or madreporite. Examples: sea lilies; feather star, Antedon. 



THE CLASS ASTEROIDEA 



Even to the inlander, starfishes are well-known animals. Among 

 the 1,100 or so species of this class, there is a great variety of color and 

 form. Most have the typical five-armed shape, but some have as many 

 as 50 arms, others as few as 4. In color, they vary from dull brown 

 to bright oranges, reds, and purples ; many are marked with lines and 

 patches of various colors. Certainly they form one of the most colorful 

 and interesting groups of animals along the seashore. 



The Anatomy of Asterias forbesi. — The anatomy of this species 

 is typical of that of the group. The living specimens show a great 

 variety of colors varying from cream or pink to orange or purple; in 



