322 



CHARACTERS OF INVERTEBRATE ANIMALS 



Order (2). Shield-gilled Snails (Aspidobranchia). — As already- 

 mentioned, these forms possess a gill or gills in which the axis 

 has a series of plates on either side. The primitive bilateral 



Fig. 185.— Strombus 

 a. Proboscis: h, notch in shell-mouth; c, eye-bearing tentacles; */, foot; ?, operculum. 



symmetry of the body has not been disturbed to the same extent, 

 for both right and left gills, auricles, and kidneys may be present. 



There are fifteen families, of which one, the Trochidce, repre- 

 sented by twenty British species, may be taken as representing 



forms with well-coiled shell, 

 the colours and markings of 

 which are often of extreme 

 beauty. Two auricles and 

 kidneys are present, but 

 only one gill. 



The Ormer {Haliotis 

 tuberculata), already de- 

 scribed (pp. 307-311), is 

 the type of another family, 

 and, as we have seen, it 

 possesses two auricles, gills, 

 and kidneys. There is good reason to believe that it is descended 

 from forms possessing a well-coiled visceral hump, covered by a 

 shell of corresponding shape, and large enough to serve as a 

 retreat into which the animal could withdraw itself at the approach 



Fig. 186. — Cowry [Cypraa] 



