520 



.MOLLl'SCA 



into the brancliial chaml>cv. lu front of the gills are two pairs of leaf-like 

 lobes, Lhe Libia! palpi, beiweeii wliich is ihe moulh. 



Tlic Xuculiibc — the most iirimiii\e of hving Aeephala have true ctenidia 

 consistiai; of an axis i;ro\vn to the bod)- and an inner and an outer row ol pll 

 leaws itv,'. :;J01, Froiu this the jilihranrh lyi>e is i'a,-;il\- deri\ed. Tlie ixill 

 lea\es gaxrw out into long thread.-, eaeti bent on itself so liuil it presents two limbs, 



pa pr ,; ^■> ::' ■) v i i:^- '■''• '' 



oh ib b »i c 



Fig. 322. — Anatome of .\)ioiIoiita, mantle, gilk and li\or of right side removed, 

 pericardvum 0])ened. a. auricle; (/(/, anterior adductor; or, anterior retractor; />, base 

 of right gills, cut a«'av; bs. branchial siphon; c, cloaca; c', cerebral ganglion; </, dorsal 

 mantle opening; /", foot; ,1,', gonad; /, intestine; ;7i, inner gill; /, liver; m. mantle; ;d, h", 

 jiarts of nephridium; no, nephridial opening; ns, nephrostonie; ob, oiUer gill; po, posterior 

 adductor; pg, pedal ganglion; pr, posterior retractor; v, ventricle; a^', ^sceral ganglion. 



a descending and an ascending. These branchial threads arc so hooked to- 

 gether that thev gi\'e the impression of a continuous leaf. In the true lamellar 

 gill the threads of the rtlibranch grow together at iiiter\als, leaxing openings, the 

 gill slits. Since there is an ascending and a descending limb, it follows th;it 

 each gill consists of an inner and an outer le;if (fig. 323), leaxing a space between 

 into which the gill slits open. This internal space in some serves to conlain the 

 young. 



The complete enclosure of tlic body in the mantle folds l\as led to a 

 degeneration of the head (a.cephala) ;ind its nonntil appendages (tentacles, 

 usually the cephalic eyes, rtidula, and s;ili\ary ghinds). Heitce there are 

 oirly two divisions iir the body, dorsally the \isceral sac aird vetilrallv the 

 foot. The tool, degenerate in matw, litis a broad sole oiil\' in Vcc- 

 tiniaihis and the Nuculida'; usually it is hatchet-shaped (rdecypoda), 

 tlntl is, comjnessed with a nninded ventral margin. It mav be enor- 

 mously expanded and conlracled again. This expansion is acconi- 



