112 



AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



visceral mass is a gill (ctenidium), made up of two elongated 

 plates (Figs. 32 and 33, K). These extend behind the visceral 

 mass below the posterior adductor, and in this region the gills of 



opposite sides are united to- 

 gether. In Fig. 33 this union 

 has been cut through in order 

 to expose part of the supra- 

 branchial chamber. At the 

 front end of the visceral mass, 

 and just beneath the anterior 

 adductor, is a transversely 

 elongated mouth (0), on each 

 side of which are two tri- 

 angular folds, the labial palps 

 (Fig. 33, Se), which are united 

 together at their bases to form 

 ridge -like anterior and pos- 

 terior lips. The exhalent 

 aperture opens out of a cloacal 

 chamber, which is the posterior 

 part of the supra -branchial 

 chamber, and is floored by the 

 united gills. The last part of 

 'the intestine or rectum,, which 

 ends in the anus (Fig. 32, Af, 

 and Fig. 33, A), runs dorsal 

 to the posterior adductor, and 

 projects into this chamber. 

 Upon either side of the vis- 



K 



Fig. 33. NERVOUS SYSTEM or ANO- 

 DONTA (from Glaus after Keber}. 

 0, Mouth; A, anus; K, gills; P, 

 foot; Se, labial palp; Gg, cerebro 



pleural ganglion ; Pg, pedal ganglion; ceral mass, near the attach- 

 Vg visceral ganglion ; G, part of t f tl m u j 



mmi-hal fflfl.nH OA' nnp mnrr nf rh+.fn 8 



genital gland ; Oe', opening of ditto ; 



Oe", opening of excretory organ. 



or female genital opening is 

 placed (Fig. 33, Oe'), and 

 close to this is a rather larger excretory or renal opening (Oe"). 



2. Skin. The most important part of this is the mantle, which 

 forms a sort of flap drawn out on either side, and secretes most 

 of the shell. The whole external surface of the mantle and 

 dorsal region of the body can add to the pearly-layer, but the 

 periostracum and prismatic layer can only be increased by the 

 thickened edge of the mantle, and hence if worn away from 



