176 



THREE CRUISES OF THE " BLAKE. 



undoubtedly the larval form of some littoral or deep-sea genus, 



which, like Amblystoma, has 

 not reached its adult condi- 

 tion, and, if subject to unfa- 

 vorable influences, continues 

 to increase in size as long as 

 it remains pelagic. 



The great development of 

 the orofans of sense and of lo- 

 comotion is a striking feature, which pelagic types share in com- 

 mon with the embryonic stages of many littoral marine animals. 

 It is true not only of the pelagic Crustacea and annelids, but even 

 of certain vertebrate embryos, in which the organs of sense are 

 developed out of all proportion to the size of the embryo. 

 Compare, for instance, the size of the eyes, of the brain, and of 

 the dorsal chord, in the young stages (Fig. 80), with their ulti- 



Fig. 79. — Rhombodichthys (Pla^usia). 



Fig. 80. — Embryo Fish, -with Lateral Organs. 



mate jDroportions in the full-grown fish. Or, as another in- 

 stance, take the Phronima (Fig. 81), which, not satisfied with a 

 set of eyes enabling it to see both laterally and downward, has 



Fig. 81. -^ Phronima. 



also an immense pair facing dorsally, so that the animal has a 

 free field of vision in all directions. 



The lateral or cephalic sense-organs of some pelagic fishes 



