SWIMMING ANIMALS. 47 



expanded "foot," are also free swimmers when first 

 hatched, the swimming being effected by means of 

 vigorous flappings of a pair of fin,s attached near the 

 head. A similar structure and habits have been re- 

 tained in the adult by the Pteropods, those small 

 translucent Molluscs, of pelagic habits, so abundant 

 in some of the northern seas, which afford a consider- 

 able proportion of the food of certain species of whales. 

 A well-known writer states that "multitudes of these 

 little things may now and then be seen on the surface 

 of the water, fluttering with their wings and glittering 

 in the sunshine, to be compared with nothing more 

 aptly than a congregation of the more dressy of the 

 Bombyx Moths." 



Although not a true swimmer, the well-known Violet 

 Snail (lanthina) is able to float on the surface of the 

 ocean, either by expanding its " foot," or by developing 

 at certain seasons a peculiar membranous raft-like 

 structure, the cells of which are filled with air, and 

 beneath which the eggs are carried. 



A totally different mode of progression through the 

 water is adopted by that group of Molluscs technically 

 known as Cephalopods, in which the head is surrounded 

 by a circle of long prehensile arms, provided with 

 adhesive suckers. This group comprises the existing 

 Cuttle Fishes, Squids, Argonauts, and Nautili, as well 

 as the extinct Ammonites (Fig. 15), and a host of other 

 fossil forms. In the free swimming forms, locomotion 

 is effected by the forcible expulsion of a jet of water 

 from a funnel situated near the head, and directed 



