6o2 



ECHINODERMATA 



as for instance in Hemiaster philippi (Figs. 250, 281), the eggs 

 are carried in some of the deeply grooved petaloid ambulacra 

 whilst in Holothuroidea they may develop in the body-cavity 

 (Phyllophorus urna), or they may adhere to the back of the 

 mother (Cucumaria crocea, Fig. 259, p. 573), or they may be 

 protected in special brood-pouches either on the ventral side of 

 the parent (Oucumaria laevigata) or on the dorsal surface (Psolus 

 ephippifer, Fig. 261). 



The majority of these cases of embryonic development have 

 been recorded from Arctic or Antarctic waters ; it appears as if 



conditions there were not 

 favourable to the larval 

 type of development. In 

 Pelmatozoa the develop- 

 ment of Antedon rosacea 

 alone is known, and 

 that is of the embryonic 

 type. 



So far, however, as 

 their mode of propaga- 

 tion is known, it may 

 confidently be affirmed 

 that the development of 

 the majority of the 



Fig. 280. Oral view of Asterias spirabilis, slightly g-pecies of Eleutherozoa 

 enlarged, showing embryos attached to the everted . , 



lips, emb, Embryos. (After Perrier. ) IS of the SeCOlld Or 



larval type. In this 

 type there is little food-yolk in the egg, and the young animal 

 or larva is forced from a very early period of development 

 to seek its own living, and hence it is usually a consider- 

 able time (from a fortnight to two months) before the adult 

 form is attained. When the embryos of different groups of 

 Eleutherozoa are compared, there is no obvious agreement in 

 structure between them ; but the larvae of the four classes of 

 Eleutherozoa exhibit with differences in detail a most remarkable 

 fundamental similarity in type, and we are accordingly justified 

 in regarding the larval development as primitive, and the 

 embryonic type as derived from it and differently modified in 

 each case. 



In the typical larval development the eggs are fertilise( 



