EMBRYOLOGY OF ECHINODERMS. 



137 



derms, appears under very different conditions in some of them. 

 There are certain Star-fishes, Ophiurans, and Holothurians, pass- 

 ing through their development under what is known as the 

 sedentary process. The eggs are not laid, as in the cases de- 

 scribed above, but are carried in a sort of pouch over the mouth 

 of the parent animal, where they remain till they attain a stage 

 corresponding to that of Fig. 168 of the Star-fish, and having 

 much the same cross-shaped outline, when they escape from the 

 pouch (as the young Ophiopholis, Fig. 184), and swim about for 

 the first time as free animals. Fig. 185 represents a cluster of 



Fig. 184. Fig. 185. 



young Star-fishes of the sedentary kind at about this period. But 

 while this mode of growth seems at first sight so different, we shall 

 find, if we look a little closer, that it is essentially the same, and 

 that, though the circumstances under which the development takes 

 place are changed, the process does not differ. The little Star- 

 fish or Ophiuran, in the pouch, becomes surrounded by the same 

 plutean structure as those which are laid in the egg ; it is only 

 more contracted to suit the narrower space in which they have to 

 move ; and the water-tubes on which the upper and lower sur- 

 faces of the body arise, the shields, spreading out into arms at the 

 corners, exist, fully developed or rudimentary, in the one as much 

 as in the other, and when no longer necessary to its external ex- 

 istence they are resorbed in the same way in both cases. This 

 singailar process of development has no parallel in the animal 



Fig. 184. Young Ophiuran which has resorbed the whole larva ; r middle plate of back. 

 Fig. 185. Cluster of eggs of Star-fishes placed over the mouth of the parent. 

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