MOLLUSCA. 175 



Wc now proceed to notice, with equal brevity, some of the 

 best known examples of the different classes of tlie encephalous 

 Mollusca, or those which have a distinct head. The classes, 

 as already mentioned (page 162), are throe iu number. 



Fig. 162.-.Hyal^a. 



The little Mollusks belonging to this order are furnished with 

 two membranous expansions, like fins or wings {Fig. 162), 

 and hence the compound term, which signifies " wing-footed," 

 points out the obvious distinguishing characteristic of the 

 class. 



There are several genera, but the species best known {Clio 

 borealis) is about an inch in length, and so abundant in the 

 Arctic seas as at times to colour the surface for leagues, and 

 to form an important supply of food to the great whale. Our 

 knowledge of its structure is principally derived from the 

 researches of Professor p]schricht, of Copenhagen. The head 

 is furnished with six retractile appendages, which are of a 

 reddish tint from the number of distinct red spots distributed 

 over their surface, and amounting on each to about 3,000.* 

 When examined under a high magnifying power, each of these 

 specks is found to consist of about twenty suckers, each 

 mounted on a footstalk, so as to be projected beyond the edge 

 of their sheath, and applied to their prey. '"Thus, to use 



• Vide Owen, page 293; Carpenter, p. 350; Jones, p. 425. 



