190 HALF AN HOUR WITH SHELL-FISH. 



vulgaris), or " Sea-pen," about six inches long, and 

 not unlike a quill-pen in appearance, hence its 

 name. 



The Pteropoda are oceanic mollusca, which take 

 their name of " wing-footed " (as the above word 

 literally means) from the fin-like expanses about the 

 head, by means of which they swim. They are 

 found chiefly in the open seas, especially in those of 

 the north, in immense numbers, the water being 

 perfectly thick with them. There they furnish the 

 principal food of the whale. Occasionally, however, 

 odd individuals of one genus, Hycdea, are cast on 

 our shores, but^their small size and glassy textures 

 conceal them from general observation. 



The next class of univalve shell-fish, however, 

 more than makes up for this scantiness. Indeed, it 

 is that to which the name of univalve most appro- 

 priately belongs, although, as we have already re- 

 marked, it contains many species which have no 

 shells at all, and others whose shell is composed of 

 many parts. As a rule, however, the shell is com- 

 posed of a single piece, often beautifully ornamented 

 in colour, and equally elegant in shape. The chief 

 distinction of the group is the great development of 

 the so-called foot, which forms a broad, flat disk, 

 with which the animals creep about* Many species 

 have what is called an operculum attached to it that 

 is, a plate composed of the same substance as the 

 shell, and so fixed that, when the mollusc has 

 withdrawn itself into its shell, it can close the 



