340 



RESPIRATION AND CIRCULATION OF CEPHALOPODA. 



the exterior by two openings one like a slit (o) for the entrance 

 of the water the other, for the exit of water and excrements, 

 formed like a tube or funnel (). By the forcible expulsion of 

 the water through this funnel, most of the Cephalopoda are able 

 to swim, tail foremost, with considerable rapidity. Each gill (b) 

 is shaped like a prolonged pyramid ; and is composed of a great 

 number of membranous lamella?, placed transversely, and fixed on 

 both sides of a central stalk. The number of gills varies ; and this 

 difference is characteristic of two great divisions, of which this 

 Class is composed. In the Poulp, Cuttle-fish, and Calamary, there 

 exists but a single pair ; whilst in the Nautilus there are two. 



951. The heart is situated between the gills, on the median 

 line of the body, and is composed of a single ventricle only (c, 

 Fig. 642). The blood from the gills flows into this ventricle 



tt> oc a et w 



FIG. 642. ORGANS' OP CIRCULATION AXU RESPIRATION IN CUTTLE-FISH. 



by branchial veins (vb\ whose openings are provided with valves : 

 it then enters the arteries (as, b), which spring from this organ, 

 and is distributed into the body. This liquid then returns into 

 a large vena cava (vc) ; which, when arrived near the heart, 

 divides itself into two branches (ab) to enter the gills ; lastly 



