3r,y 



MOLLUSC A. 



chamber and the larger lower respiratory cavity. Into the cloaca 

 empty tlie anus and the water which has passed over the gills; it 

 opens to the exterior through the excurrent siphon. The inciirrent 

 siphon leads into the branchial chamber. In front of the gills 

 are two more pairs of leaf-like lobes, the labial palpi, between 

 which is the mouth. 



The gills are variouslj' developed. The Nuculidfe — the most primitive 

 of living Acephala — have true ctenidia consisting of an axis grown to 

 the body and an inner and an outer row of gill leaves (fig. 355). From 

 this the tilibranoh type is easily derived. The gill leaves grow out into 



Fig. 3.52. — Pro.iection of sections shown by the arrows in fig. 351. ^l, ?)-, upper and 

 lower limbs of nephridium (organ of Bo.ianus) ; w, intestine ; e, nepliridiopore : 

 /7(, foot; (7, gonad; 'i^ ventricle surrounding the intestine : A^, auricle ; /l-i, k". inner 

 and outer gill laraellse ; ^, hinge ligament: jh, mantle; n, cereijro-viscerai com- 

 missure ; sjj, nephrostome; %\ venous sinus. 



long filaments, each bent on itself so that it presents two limbs, a descend- 

 ing and an ascending. These branchial threads are so matted togetber 

 that they give the impression of a continuous leaf, fn the true lamellar 

 gill the threads of the filibranch grow together at intervals, leaving open- 

 ings, the gill slits. Since there is an ascending and a de.scendiug limb, it 

 follows that each gill consists of an inner and an outer leaf (fig. 353), leav- 

 ing a space between into which the gill slits open. This internal sp.icc in 

 some serves to contain the young. 



The complete enclosure of the body in tlie mantle folds has 

 led to a degeneration of tlie liciid and its normal appendasjes 



