110 



NATURAL HISTORY. 



( i„ reference to our figures the appropriateness of the name Pteropods 

 (which means wing-footed) can readily be seen, for the wing-like projec- 

 tions (.n either side of the body are but the lateral expansions of the foot 

 f the animal. By the aid of these wings they swim — one might almost 

 sa , fly — through the water, some progressing in a steady manner, others 

 jerking this way and that after the most erratic fashion. All are carniv- 

 orous mihI rapacious; and one begins to realize the amount of life in the 

 i when lie considers the immense numbers of individuals necessary to 

 keep the immense schools of brit alive. 



The brit and some of its allies have no shell, but others of the Ptero- 

 pods are shelled. In some cases the 

 shell is thick and opaque, in others 

 hyaline or glassy ; sometimes it is 

 straight and needle-shaped, sometimes 

 coiled in a spiral, or, as in the Hyalea 

 figured, almost globular. This form 

 is abundant in tropical and semi- 

 tropical seas, and is the genus most 

 frequently represented in collections. 

 It swims rapidly, but in a jerking 

 manner, through the water, remind- 

 ing one somewhat of the erratic 

 flight of some of the smaller but- 

 terflies. Another genus (Cymbulia) 

 has a cartilaginous shell, shaped much like a slipper, and studded all 

 over tin- surface with projecting points. 



Fig. 104. — Hyalea. 



CEPHALOPODS. 



The squids and cuttle-fishes form the group of cephalopods, the most 

 highly developed of all the molluscan series. Here we find animals with 

 eyes worthy of comparison with those of the human species ; here the brain 

 is partially enclosed iii a cartilaginous capsule which at once suggests the 

 skull (if the vertebrates; here, too, the circulatory system acquires a high 

 development. Anion-' the squids we also find the largest of the mol- 

 luscs. — huge monsters which seem to lend countenance to the old stories 

 of krakens, devil-fish, and the like; but it is to be remembered that mere 

 size is no criterion of rank in the animal kingdom. 



There are two great groups of cephalopods: in one there is but a 

 Mimic pair of gills, in the other there are two pairs; and these features are 

 correlated with a number of others which need not be discussed here, as 



