Chap. XIV.] 



OF CR i NO IDS. 



535 



in less highly developed forms, but only transitory in 

 the higher ; these, however, differ in different orders, 

 and are beyond our consideration here. 



Finally, it is to be borne in mind that some Crus- 

 tacea leave the egg in a form essentially similar to that 

 of their parent; of such forms the crayfish is an example. 



Some remarkable larval forms obtain among the 

 Ecliinodermata, and the 

 wide distribution of species 

 which, when adult, are capable 

 of but a slight amount of loco- 

 motion, must be ascribed to 

 their possession of free-swim- 

 ming ciliated larvae. The most 

 instructive examples are pre- 

 sented by the Comatulidse, 

 which are members of the 

 group Pelmatozoa, but are 

 stalked in their larval stages 

 only, during which, therefore, 

 they have a certain resem- 

 blance to the permanently- 

 stalked Pentacrinus. After 

 passing through a short period 

 of free existence, in which the 

 cilia are arranged in four 

 transverse bands (Fig. 220), 

 and during which two sets of 

 five plates and a short calcareous stern become de- 

 veloped, the larva loses its ciliated bands, and becomes 

 fixed by the stalk (Fig. 221 ; A) ; at the free end of this 

 stalk the arms become developed, and below the cup- 

 like portion (calyx) there appear the jointed pro- 

 cesses which are known as the cirri. The calyx and 

 the top joint of the stem break away from the rest, 

 and we get the Comatulid which is capable of a certain 

 amount of locomotion. 



Fig. 220. Torsal view of the 

 Larva of the Common 

 British Feather-star (An- 

 tedon rosacea) ; x 20. 

 (After Wyville - Thom- 

 son.) 



