232 HIGH SCHOOL ZOOLOGY. 



in tlie skin of various small fish, where they undei-go a resting 

 stage before they attain their adult form. 



17. The Unionidae belong to an order Asphoniata distinguished by the 

 absence of tubular siphons j the oyster {Ostrea) and scallop {Pecten) also 

 belong here, but they have only one (the posterior) adductor muscle, and 

 no foot. Intermediate between these types is the sea-mussel {Mi/tilus), 

 in which the anterior adductor and the foot are small. These forms are 

 attached, not by the shell, but by horny "byssus " threads secreted by 

 the foot. 



18. A second order Siphoniata is formed for those in which the mantle 

 cavity is closed] except for the tubular siphons behind, and an aperture 

 for the foot in front. Numerous minute fresh-water forms {Cyclas, 

 Pis'uUmn) belong here, but the bulk of the forms are marine. When the 

 siphons are long, and can be retracted into the shell, there is a corres- 

 ponding mark within the shell (F g. 157), as in the Sea-clams {Mya and 

 Venus). Some of the forms burrow in sand, others in rocks (Pholas), or 

 timber {Teredo). 



19. The higher division of the Mollusca (tlie Cephalophora) 

 presents a much greater range of form than do the Acephala; 



four Classes are recognized, each of them 

 exhibiting important modifications of the 

 typical Molluscan structui-e. Most of the 

 species belong to one of these, the Gas- 

 Fig. 157. tropoda, called so on account of the de- 



Tellma grcenlandica. 



velopmenfc of the foot into a locomotive 



or-gan, generally a flat creejiing surface, occupying the ventral 



aspect of the body. 



Very few Gastropods retain the bilateral symmetry which we 

 see in the Acephala ; the most primitive forms — the Chitons — 

 do, (a group, which of all the Mollusca, comes nearest to the 

 Vermes), but in most a distinct asymmetry is present. This 

 depends upon a separation of the vegetative from the animal 

 organs, and the gi-ouping of the former into a visceral mass or 

 hump, above the head and foot, which are closely united (Fig. 1 58). 

 The visceral mass is protected by a shell (similar in structure 



