CIRCULATION IN CEPHALOPODS. 



21 



the venous is changed into arterial blood. The number ot' 

 branchise varies, and this difference is characteristic of the two 

 great natural divisions, of which this class is composed. In 

 some there is but a single pair, while in others two pairs of 

 branchia} are found. For this reason the first have been called 

 cephalapoda dihranchiata (from the Greek dis, two, and 

 hragchos, branchia or gills, — two-gilled), and the last, cephalo- 

 poda teirahranchiata (from the Greek, tetras^ four, and bragchosj 

 branchia, — -four-gilled). 



4. The heart is situated between the branchise on the middle 

 line of the body, and consists only of a single ventricle 

 {fg. 9, c): the blood reaches it from the branchise by the 



vv av a cs vv 



Fig. 9. ORGANS OF CIRCULATION AND RESPIRATION IN THK CUTTLE-FISH. 



Explanation of Fig. 9. — The organs of circulation and respiration of the 

 cuttle-fish ; — c. the aortic heart, the upper extremity of which is continuous 

 with the superior aorta, which distributes the blood to the head, &c. ; — h. 

 branches of this vessel ; — a. the inferior aorta, which has a bulb at its origin, 

 and soon divides into two branches (u, v) ; — vc. tlie vena cava, the parietes 

 of which are covered by the spongy bodies (cs) ; — av. veins of the viscera, 

 going to empty into the two branches of the vena cava ; — ch. venous sinus 

 or branchial heart ; — s. swelling at the base of the branchial arteries ; — hr, 

 branchiiE ; — ab. branchial artery ; — vb. branchial vein ; — bu. bulb of the 

 branchial veins, situate near the termination of these vessels in the heart. 



4, What is the character of the heart in cephalopods ? What are the 

 peculiarities of the circulation ? 



