THE PELAGIC FAUNA AND FLORA. 



175 



Near the continental lines the pelagic fauna is reinforced by 

 numberless pelagic embryos, representing nearly every type of 

 marine animal. The polyps (Fig. 63), acalephs, echinoderms 

 (Figs. 64, 64a), mollusks (Figs. 65, 66, 67), and articulates (Figs. 

 C3, 69, 70, 71, 72, 73, 74, 75, 76), of our coasts, although living 

 upon the bottom and on the shores when adult, yet pass a por- 

 tion of their earher life as pelagic forms. The larvae and young 

 of many of these types swarm during the breeding season, and 

 often find then* way to a considerable distance from the shore. 

 With them are associated the eggs (Fig. 77) and embryos of 

 a number of our migratory and shore fishes. The 

 study of all this pelagic life is very recent. There 

 are many types of which the life history is un- 

 known, so that it is often diJEicult to determine 

 whether an animal is merely the young of some 

 well-known littoral form or a true pelagic type. 

 Indeed, many of these free-swimming animals, de- '^" 



scribed at first as strictly pelagic, have subsequently proved to 

 be only embryonic stages of well-known species. 



Fig. 77. — Pe- 

 lagic Fish Egg. 



Fig. 78. — Plagiisia. i. 



It is probable that among the larval forms of many pelagic 

 animals there is, under certain conditions, a retardation of de- 

 velopment similar to that known among batrachians. The 

 larval stage continues in Plagusia (Fig, 78) and Phyllosoma 

 long after the time when the embryo should have passed into 

 Rhombodichthys (Fig. 79) and Palinurus. Leptocephalus is 



