CLASS OF THE CEPHALOPODA. 



465 



interior communicates with the exterior by two openings. 

 The one (o) in the form of a fissure serving for the entrance 

 of the water; the other, prolonged into a tube or funnel (t), 

 serving for the escape of the water and of the residue of the 

 food. Each gill (b) has the shape of an elongated pyramid, 

 and is composed of a great number of membranous lamellae, 



Ar 



Fig. 448. Organs of Kespiration and Circulation.* 



placed transversely, and fixed on either side of the median 

 stalk. The number of gills varies, and this diflference is 

 characteristic of the two great natural divisions of which the 



* c, the aortic heart, the superior extremity of which is continuous with 

 the superior aorta (as), distributing the blood to the head, &c. ; b, the 

 branches of this vessel ; a, the inferior aorta, presenting a bulb at its origin, 

 and soon dividing into two branches (vv) ; cc,venacava,whosewalls are covered 

 by the spongy bodies (en) ; vv, veins of the viscera proceeding to open into 

 the two (branches of the vena cava ; cp, venous sinuses or branchial hearts ; 

 s, enlargement of the base of the branchial arteries ; br, gills ; ab, branchial 

 artery ; vb, branchial vein ; bit, bulb of the branchial veins situated near the 

 termination of the vessels in the heart, and constituting the auricles." 

 H H 



