MOLLUSCA 127 



1. Filibranchia, with two pairs of platelike gills, the 

 filaments being V-shaped, usually two adductor muscles 

 of which the anterior is often the smaller or may even 

 be absent, sea mussel (Mytilns) (Fig. 85). 



2. Pseudo-lame llibranchia, with gills showing vertical 

 folds, a single, large (posterior) adductor muscle, the 

 shell frequently inequivalve, oyster (Ostrea) (Fig. 242), 

 scallop (Pec fen), pearl oyster (Meleagrina) (Fig. 84). 



3. Eulamellibranchia, with gills smooth or vertically 

 plaited and with two adductor muscles of equal size, 

 fresh-water mussel (Unio and Anodontd), cockle (Car- 

 dium) (Fig. 87), quahog (Venus), shipworm (Teredo), 

 and common clam (Mya). 27 



CLASS 2. : Amphineura 



The animals in this class were formerly placed among 

 the Gastropoda, but are now considered to be sufficiently 

 distinct to be grouped by them- 

 selves. They are bilaterally 

 symmetrical, elongated mollusks, 

 with a shell consisting of eight 

 separate pieces, or else entirely 

 lacking. The mantle is not di- 

 vided into paired lobes as in the 

 bivalves. Chiton, a sluggish ani- 

 mal with the habit of the limpet, 



FIG. 87. Cockle (Cardznm cos- 

 1S One Of the beSt-knOWn forms tatuni) \ one third natural size. 



(Fig. ioo). The shell-less mem- Chinaseas - 



bers of the class are the lowest in organization of all 



of the mollusks. 



CLASS 3. Gastropoda 



The snails are, with rare exceptions, all univalves. 28 

 The body is coiled up in a conical shell, which is usually 

 spiral, the whorls passing obliquely (and generally from 



