210 



METAZOAN PHYLA 



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 ophiopluteus, that of a sea urchin as an echino- 

 pluteus, and that of a sea cucumber as an auricularia. 



242. Behavior. — In the echinoderms the nervous 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 readi- 

 ness 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 coordination. Habit 

 formation has been referred to in the preceding 

 chapter. 



243. Color. — The echinoderms when living are 

 very strikingly colored, but color may mean 

 nothing 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. 



244. Occurrence and Economic Importance. 



Fig. 123. — Rhizocri- 

 nus lofotensis Sars. {From 

 Bather, in Lankester, 

 "A Treatise on Zoology," . riij. u u- 



after Wyviiie Thomson, Becausc of the possessiou of skeletons by echi- 

 by the courtesy of A. and C. noderms their parts have been preserved in 

 ^' abundance in rocks from very early times, and 



many limestone strata are filled with such fossils. Among these 

 are spines and plates, sections of the stems of crinoids, and in some 

 cases the crinoid body, very nearly perfect. At the present time 

 echinoderms are widely distributed and abundant in all seas. The 

 phylum is one of the few which has always been exclusively marine. 



