354 



ANIMAL LIFE-HISTORIES 



LIFE-HISTORIES, &c, OF HEDGEHOG-SKINNED ANIMALS 



(ECHINODERMATA) 



In most members of this group the eggs hatch out into larvae, 

 which are often of remarkable shape and extremely unlike the 

 parent forms, the characters of which they ultimately acquire by 

 passing through a complicated series of changes constituting a 



Fig. 873. Life-history of a Brittle-Star 



m, Mouth; /.., intestinal aperture; c.b., ciliated band; sk, calcareous skeleton. A-C, Stages in development 

 ofPluteus. D, Young Brittle-Star developing on the Pluteus. Enlarged. 



metamorphosis. Each subdivision of the Echinoderms is charac- 

 terized by a particular kind of larva, and we may take the Brittle- 

 Stars (Ophiuroidea) as one of the most interesting illustrations. 

 The just-hatched young is oval in shape and bilaterally sym- 

 metrical, there being a well-marked distinction between front 

 and back ends, and also between dorsal and ventral surfaces. 

 Both mouth and intestinal aperture are on the ventral surface, 

 and there is a ciliated band by means of which swimming is 



