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 (t). Each gill (I) is shaped like a 

 prolonged pyramid ; and is composed of a great number of mem- 

 branous lamellag, 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. 



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

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

 Fig. 542). The blood from the gills flows into this ventricle 



vv av a cs 

 FIG. 542. ORGANS OF CIRCULATION AND RESPIRATION IN CUTTLE-FISH. 



by branchial veins (vb), whose openings are provided with valves; 

 it then enters the arteries (as, &,) 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, 

 these vessels, when arrived at the base of the respiratory organs, 



