34 MARINE AOTMALS OF MASSACHUSETTS BAY. 



EMBRYOLOGY OF CTENOPHOR^. 



All the Ctenophorae are reproduced from eggs, these eggs 

 being so transparent that one may follow with comparative ease 

 the changes undergone by the young while still within the egg 

 envelope. Unfortunately, however, they are so delicate that it is 

 impossible to keep them alive for any length of time, even by 

 supplying them constantly with fresh sea-water, and keeping 

 them continually in motion, both of which are essential conditions 

 to their existence. It is therefore only from eggs accidentally 

 fished up at different stages of growth that we may hope to ascer- 

 tain any facts respecting the sequence of their development. 

 When hatched, the little Ctenophore is already quite advanced. 

 It is small when compared with the size of the egg envelope, and 

 long before it is set free, it swims about with great velocity with- 

 in the walls of its diminutive prison (Fig. 35). The importance 

 of studying the young stages of animals can hardly find a better 

 illustration than among the Ctenophorae. Before their extraor- 

 dinary embryonic changes were understood, many of the younger 

 forms had found their way into our scientific annals as distinct 

 animals, and our nomenclature thus became burdened with 

 long lists of names which will disappear as our knowledge ad- 

 vances. 



The great size of their locomotive flappers in proportion to the 

 rest of the body, is characteristic of the young Ctenophorae. 

 They seem like large paddles on the sides of these tiny trans- 

 parent spheres, and, owing to their great power as compared with 

 those of the adult, the young move with extraordinary rapidity. 

 The Pleurobrachia alone retains its quickness of motion in after 

 life, and although its long graceful streamers appear only as short 

 stumpy tentacles in the young (Fig. 34), yet its active little body 

 would be more easily recognized in the earlier stages of growth 



son with those of the Bolina. The other, the Mertensia, is closely allied to Pleuro- 

 brachia ; it is exceedingly flattened and pear-shaped. This species was discovered 

 long ago by Fabricius, but had escaped thus far the attention of other naturalists. 

 (A. Agassiz.) 



