206 



METAZOAN PHYLA 



tube feet are tentacle-like and without ampullae. Crinoids are found 



mostly at moderate depths, but a few belong to the deep-sea fauna. 



They may be free-swimming, when they are known as feather stars 



(Fig. 115). 



239. Reproduction. — The reproduction of all echinoderms is similar 



to that of the starfish. They all develop a bilaterally symmetrical 



swimming larva, and all undergo metamorphosis. 

 The larvae of the different classes resemble each 

 other in a general way, but each is quite distinct. 

 That of a starfish is known as a bipinnaria or 

 brachiolaria, that of a brittle star as an ophioplu- 

 teus, that of a sea urchin as an echinopluteus, and 

 that of a sea cucumber as an auricularia. 



240. Behavior. — In the echinoderms the ner- 

 vous system does not seem to control the muscles 

 as does a centralized system in other animals but 

 simply maintains a certain tonus, a condition 

 accompanied by readiness to respond to stimuli. 

 The response itself is, generally speaking, direct. 

 Pedicellariae react to the presence of an object 

 in contact with the skin near them by seizing it 

 in their jaws and either holding it or so moving 

 as to carry it away from the point of contact. 

 In this way they serve to keep the surface clean, 

 especially over the dermobranchiae. A strong 

 stimulus results in locomotor impulses being 

 carried to the tube feet through the nervous 

 system, which in this way functions in coordina- 

 tion. Habit formation has been referred to in 

 the preceding chapter. 



241. Color. — The echinoderms when living are 

 very strikingly colored, but color may mean noth- 

 ing in the discrimination of species, some forms 

 quite regularly exhibiting a particular color but 

 others showing marked variations. Almost all 

 colors are represented in the group, various 

 shades of red and orange being common. There 



are also varying tints from cream to almost white, and innumerable 

 shades of buff, brown, green, blue, and purple, some being almost black. 

 When a group of sea cucumbers are seen on the bottom with their 

 tentacles spread and possessing varied and brilliant hues, they appear as 

 striking as a bed of variously colored flowers. 



242. Occurrence and Economic Importance. — Because of the posses- 

 sion of skeletons by echinoderms their parts have been preserved in abun- 



FiG. 114. — Rhizocri- 

 nus lofotensis Sars. {From 

 Bather, in Lankester's 

 "A Treatise on Zoology," 

 after Wyville Thomson, 

 by the courtesy of A. and C. 

 Black.) X 4H- 



