3S6 



ANIMAL LIFE-HISTORIES 



life-history as Brittle-Stars, being hatched out as Pluteus larvae, 

 though these differ in some respects from the kind already- 

 described. And in this case the mouth and gullet of the larva 

 do not become those of the adult, which has to develop a second 

 edition of these parts. Some Sea-Urchins resemble the excep- 

 tional Brittle- Stars already mentioned in having given up the 



larval type of develop- 

 ment. And here also 

 brood-pouches are pre- 

 sent, in which the eggs 

 are sheltered, as in the 

 species represented in 

 fig. 874, where parts 

 of the areas from which 

 the tube - feet project 

 have become deep 



which serve 

 this purpose. Some 

 Sea-Urchins hollow out 

 dwellings for themselves 

 in limestone rocks by 

 means of their spines 

 (fig. 875), and others 

 attain the same end by 

 burrowing in the sand 

 (fig. 876). But as in 

 both cases the eocrs 

 hatch into free-swim- 

 it cannot be said that these dwellings serve as 

 nurseries. 



Ordinary Star-Fishes [Asie^-oidea) as a rule present another 

 type of indirect development, in which the larval forms (Bipinnaria 

 and Brachiolaria) are in many respects unlike the Pluteus. Here 

 again we find cases of direct development associated with arrange- 

 ments for care of eggs or young. A remarkable instance is 

 afforded by the members of one family {^Pterasteridcs), for in 

 these the upper side of the body forms the floor of a kind of 

 pouch in which the young pass through their development, while 

 the roof of this space is constituted by a special membrane sup- 

 ported on spines, and pierced by numerous small holes and a 



Fig. 875. — Sea-Urchins burrowing in Limestone 



larvae, 



