PELAGIC ZONE— SURFACE LIFE 



451 



p. 425) that a great many species lay floating eggs, and these, 

 together with the transparent larvae that hatch out of them, 

 belong to the plankton fauna. Primitive Vertebrates {Proto- 

 chordata) are abundantly represented by certain Sea-Squirts or 

 Ascidians {Urochorda), including some little tadpole-shaped forms 

 {Appendicularia, &c.). Barrel Ascidians {Doliolum), Salps {Salpa), 

 and Fire-Cylinders {Pyrosoma), all of which have received notice 



Fig. 1306. — Wjng-Footed Snails [Pieropoda). a, Cuvierina; B, Clio; c, Creseis; D, shell of Cleodora; E, Limacina; 

 F, Clione; G, Halopsyche: H, front part of Pneumoderma, with hook-bearing tubes and groups of stalked suckers, for 

 securing prey, a-e. Shell-bearing forms: F-H, shell-less forms. 



in earlier sections (see vol. i, p. 299; vol. iii, pp. 38 and 422; and 

 p. 106 of present volume). 



Plankton Molhtscs [Moihisca). — The beautiful Violet- Snail 

 {lanthmd), with its egg-raft, belongs here, also the members of 

 the remarkable and diversified group of Fin-Footed Snails [Hetero- 

 podd), and a curious little Sea-Slug [Phyllirhoe), which is flattened 

 from side to side (see vol. iii, pp. 34 and 36). Far more 

 characteristic than these, however, are the little Wing- Footed 

 Snails {^Pteropoda, fig. 1306), which are often found associated 

 in vast shoals, affording an important contribution to the bill of 

 fare of animals so laree as Whales. 



