CLASS PTEROPODA. 69 



above characters the ones which the Whelk shares Avith 

 the other Gasteropods, and which therefore characterise 

 this class of shell-fish, are only the following : The head 

 is distinctly marked out from the rest of the body ; the 

 mouth is furnished with a peculiar toothed apparatus or 

 "tongue;" the "foot" is used for locomotive jDurposes, 

 having the form (not always) of a broad, flattened, mus- 

 cular disc ; and the body is not enclosed in a bivalve shell. 

 Non-essential, though very common, is the character that 

 the soft body is protected by a shell which is " univalve " 

 or consists of a single piece. 



CLASS PTEROPODA. 



This class of animals contains minute shell-fish, which are 

 found swimming in the open ocean far from land. They 

 derive their name of FierojMda from the fact that the head 

 is furnished with two wing-like fins, by means of which 

 the animal swims (Greek, p<ero?i, wing ; podes, feet). We 

 may select as the type of this class the well-known Hyalea 

 tvidentata of the Mediterranean and Atlantic, but we shall 

 give merely a brief outline of the most important points 

 in its organisation. 



The animal of Hyalea tvidentata (fig. 30) is enclosed in 

 a small, yellowish -brown, semi-transparent shell, which 

 may be regarded as composed /^rr>. ^ ^ ^rr\ 



of a back and front plate, (C, ^^^^T" ,;:p 



united to one another more y'' ^.i:J;;^^^^^;;;;;>r^'■-- 

 or less completely. The back fV^^J^ ^ 

 plate is nearly flat, and is pro- /a>\^^£* \ 

 longed in front so as to form v/^^f^\\ 

 a sort of hood. The front ]H y v\ 

 plate is strongly rounded and / | 

 globular. Behind, the shell is „ , . .^ . . . . 



° 1 1 • i ii • Fife, -io.— Hyalea tndentata, showing 



prolonged mtO three spines, tiie shell and the lateral fins attached 



which arise from the line to the sides of the head (//). 

 where the two plates of the shell unite with one another. 

 In front the two plates leave a small aperture, through 

 which the animal can protrude its head at will ; and at the 



