GROWTH, MIGRATIONS, FOOD AND HABITS II 9 



The habits of the fishes which feed upon the lower animals 

 again are necessarily related and adapted to the mode of life of 

 these lower animals. The principal different kinds of marine 

 creatures which are not fishes are the Echinoderms, that is to 

 say, starfishes, sea-urchins and the like, Worms, Molluscs, and 

 Crustaceans. In all these classes most of the larger kinds live 

 on the sea-bottom, and have no powers of swimming, or very 

 insignificant powers. The most important exception to this 

 general rule is formed by the squids and cuttle-fishes, which are 

 swimming molluscs with reduced shells. The echinoderms, or 

 spiny-skinned animals, usually remain exposed on the surface of 

 the ground, but many of the worms, molluscs, and crustaceans 

 conceal themselves as much as possible either by burrowing in 

 soft ground, by building tubes or coverings, or by hiding among 

 stones, rocks, or sea-weed. Many of these animals, chiefly 

 among the worms and molluscs, are quite stationary ; in the 

 large and abundant class of bivalve molluscs such as the mussel 

 and oyster, scallop, razor-shell, &c., the power of movement is 

 usually extremely limited. All these classes of animals, which 

 inhabit the bottom of the sea in very great numbers, form the 

 food of bottom fishes. The fishes continually seeking these 

 creatures on the ground beneath the water may be compared to 

 the flocks of starlings and other birds which feed upon the 

 worms and insects, snails and slugs on land. Different fishes 

 feed principally on different classes of lower animals, but no 

 kind of fish confines itself exclusively to one class of prey. Thus 

 the haddock and cod feed principally on Crustacea, but devour 

 also large numbers of molluscs. The gray gurnard also feeds 

 principally on Crustacea, and likewise the large-mouthed long 

 rough dab. The principal food of the plaice is composed of 

 molluscs and worms, the former usually predominating : the sole 

 and lemon dab live chiefly on a diet of worms. In each class of 

 the lower animals there are certain common and abundant forms 

 which are of chief importance as food for the valuable fishes. 

 Among the Crustacea the hermit-crabs are devoured in very large 

 numbers. These crabs, as is well known, live in the empty 

 shells of molluscs. There are several species of them differing 

 in size. The largest, the common hermit-crab (Fig. 6^,) is 

 usually found in shells of the common whelk. The shells are 

 not swallowed by the fish, but the crab is dragged out of its 



