MOLLUSC A. 



chamber and the larger lower respiratory cavity. Into the cloaca 

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

 opens to the exterior through the excurrent siphon. The incurrent 

 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 variously developed. The Nuculidse 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 filibranch type is easily derived. The gill leaves grow out into 



...*' 



FIQ. 352. Projection of sections shown by the arrows in fig. 351. 6 1 , b 2 , upper and 

 lower limbs of nephridium (organ of Bojanus) ; rf, intestine ; e, nephridiopore ; 

 /M, foot ; g, gonad ; h 1 . ventricle surrounding the intestine ; h 2 , auricle : fc 1 , fc 2 , inner 

 and outer gill lamellae ; J, hinge ligament ; m, mantle ; n, cerebro-visceral com- 

 missure ; sp, nephrostome ; v, 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 together 

 that they give the impression of a continuous leaf. In 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 descending limb, it 

 follows that each gill consists of an inner and an outer leaf (fig. 352), leav- 

 ing 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 and its normal appendages 



