184 



THE BOOK OF NATURE STUDY 



wise, how ? why ? You find Ascidians nondescript, water- 

 bag-like creatures fixed to the stalk of one of the big sea- weeds ; 

 they began life as little tadpole-like free swimmers, with all the 



ZZZ^ 



L.L 



FIG. 105. Gunnel or butterfish (Pholis gunnelhis}. 

 PF, small pelvic fins. 



LL, lateral line ; 



essential characters of a back-boned animal (see Fig. 101) how 

 and why has this come about ? So one might continue page after 

 page, for there is no end to the shore's unsolved problems. 



THE OPEN SEA 



The conditions of life in the open sea are sharply contrasted 

 with those of the shore. They are on the whole very much 



easier. There can be no 

 lack of room, there is abun- 

 dant sunshine without risk 

 of drought, there is an 

 evener life throughout the 

 day and throughout the 

 year than is to be found 

 elsewhere except in the 

 abysses of the deep sea, 

 there is an inexhaustible 

 supply of minute unicell- 

 ular Algae, which form the 

 basal food material. 



Many of the pelagic or 

 open-sea animals are active 

 swimmers, e.g. jelly-fishes 

 (Fig. 108) and cuttle-fishes, 

 and are included under 



FIG. 1 06. Velella, one of the Siphonophora, about 

 natural size. It is a characteristic pelagic 

 animal which sometimes drifts on to British 

 coasts from the Atlantic. It is a colony of 

 numerous individuals, with a beautiful trans- 

 parent triangular sail (V) on its upper surface, 

 and a flattened chambered float beneath. In the 

 centre of the under surface there is a large nutri- 

 tive individual or zooid (G), around the margin 

 of the disc there is a fringe of simple, finger- 

 like sensitive zooids ; between these and the 

 centre there are numerous reproductive zooids. 

 Fleets of these transparent blue velellas are 

 sometimes seen on the surface of the Mediter- 

 ranean, and thousands are often found wrecked 

 on the shore. 



the term Nekton ; a large 

 number do little more than drift in the currents, e.g. Radiolarians, 

 many small Crustaceans, the larvae of Echinoderms, Crustaceans, 

 and Molluscs, and these are included under the title Plankton. 



