1 90 Elementary Zoology. 



simple. Examples : with two adductor muscles, Anodon, 

 Mytilus (the marine edible mussel) ; with one adductor muscle, 

 Ostrea (oyster), Pedcn (scallop), etc. 



Class 3. CEPHALOPHORA. 



These Molluscs have a head and a distinct radula (only 

 wanting in rare cases such as the Nudibranch Doriopsis). The 

 body is symmetrical, and the shell is single. The symmetry of 

 the body is more perfect in some than in other forms. In 

 those in which it has advanced furthest there is but one gill 

 instead of two, and one nephridium. In others there are two 

 nephridia, but one is smaller than the other. The generative 

 organs are generally complicated, but are simpler in the forms 

 which have most nearly retained the primitive bilateral 

 symmetry. The Cephalophora may be divided into the follow- 

 ing subclasses : Prosobranchs, Heteropods, Opisthobranchs, 

 Pteropods, Pulmonates, and Scaphopods. We shall briefly 

 consider the characters of these subclasses. 



1. Prosobranchs. — Of this group, which are characterized by 

 the fact that the gills lie in front of the heart, the simplest forms 

 are those which have been termed the " Zygobranchia ; " in 

 them there is so much of the bilateral symmetry retained that 

 the gills are two — save in the limpet {Patella), where there is a 

 nearly complete circle of gills — and the nephridia are also 

 double, though one may be larger than the other. Two auricles, 

 moreover (as in Lamellibranchs), exist in Haliotis. In these 

 forms there are no special generative ducts; the nephridia serve 

 as such. The more advanced Prosobranchs are termed the 

 " Azygobranchia." They have but one gill, the right ; only one 

 nephridium, the left, is retained. The sexes are separate, and 

 there are special generative ducts. To this division belong the 

 fresh- water pond-snail, Paludina (Fig. 82), and the bulk of 

 marine univalved " shell fish," such as the whelk {Buccinum), 

 cowrie-shell {CyprcBa), Conus, the periwinkle {Littorina), the 

 purple-producing Murex, etc. 



2. Heteropoda. — By some associated with the last division. 



