320 



MOLLUSCA 



into the branchial chamber. In front of the gills are two pairs of leaf -like 

 lobes, the labial palpi, between which is the mouth. 



The Nuculidae the most primitive of living Acephala have true ctenidia 

 consisting of an axis grown to the body and an inner and an oncer row of gill 

 leaves (fig. 326). From this the filibranch type is easily derived. The gill 

 leaves grow out into long threads, each bent on itself so that it presents two limbs, 



pa 



ar 



- f 



ob ib b m g 



FIG. 322. Anatomy of Anodonta, mantle, gills and liver of right side removed, 

 pericardium opened, a, auricle; aa, anterior adductor; ar, anterior retractor; b, base 

 of right gills, cut away; bs, branchial siphon; c, cloaca; eg, cerebral ganglion; d, dorsal 

 mantle opening;/, foot; g, gonad; /, intestine; ib, inner gill; /, liver; m, mantle; ', n'-, 

 parts of nephridium; no, nephridial opening; ns, nephrostome; ob, outer gill; pa, posterior 

 adductor; pg, pedal ganglion; pr, posterior retractor; v, ventricle; vg, visceral ganglion. 



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

 gether that they give the impression of a continuous leaf. In the true lamellar 

 gill the threads of the filibranch grow together at intervals, leaving openings, the 

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

 each gill consists of an inner and an outer leaf (fig. 323), leaving a space between 

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

 young. 



The complete enclosure of the body in the mantle folds has led to a 

 degeneration of the head (acephala) and its normal appendages (tentacles, 

 usually the cephalic eyes, radula, and salivary glands). Hence there are 

 only two divisions in the body, dorsally the visceral sac and ventrally the 

 foot. The foot, degenerate in many, has a broad sole only in Pec- 

 tuncidus and the Nuculids; usually it is hatchet-shaped (Pelecypoda), 

 that is, compressed with a rounded ventral margin. It may be enor- 

 mously expanded and contracted again. This expansion is accom- 



