PHYLUM ECHINODERMA 



181 



oral disc to the mouth opening. In the case of distinctly dicho- 

 tomously branched arms, there may be but a single groove 

 across the disc to represent each pair of arm branches. The 

 grooves are ciliated and serve as channels along which food 

 particles are borne to the mouth opening. Ambulacra line the 

 margin of the groove along each arm but they function as 

 tactile organs for they lack suckers and ampullae. The nerv- 

 ous and circulatory systems follow the course of the ambulacral 

 system in muchthe same manner as already described for other 

 echinoderms. 



In its normal position, the crinoid directs the oral surface and 

 tentacles upward. Its position is thus the reverse of that 

 normally assumed by the echinoids, asteroids, and ophiuroids. 



The full course of development is known for but a single species, 

 Antedon rosacea. The germ cells are dehisced from the pinnules 

 of the arms to the outside of which the eggs become attached. 

 Fertilization of the heavily yolk-laden 

 eggs is followed by cleavage resulting 

 in the formation of a free-swimming 

 ciliated larva, in which there is no 

 communication between the mouth 

 and the stomach. In this respect, 

 the larva resembles that of some of 

 the highly modified larvae of echino- 

 derm groups rather than the typical 

 free-living echinoderm pluteus, auri- 

 cularian, or bipinnarian. Calcareous 

 plates begin to make their appearance 

 early in the development of the larva. 

 After a few days of free-swimming arms to mouth. (After King- 



n J sley in Hertwig's Manual, court- 

 existence, the larva becomes faxed esy of Henry Holt and Co.). 



(Fig. 88) and undergoes a compli- 

 cated series of changes which lead to the differentiation of 

 calyx and stalk. The larva in this condition is said to be in the 

 Pentacrinus stage. In later development, there is consider- 

 able resorption of skeletal plates characteristic of the larva. 

 Ultimately, the animal becomes detached from the stalk and 

 capable of independent movement. 



Fig. 89. — Oral area of crinoid 

 (Antedon), showing by dotted 

 lines the course of the intestine 

 from mouth (m) to anus (a) ; g, 

 ciliated grooves leading from 



