96 



COM PA BA TI VE ANA TOM Y 



CHAP. 



D. Cephalopoda. 



The gills of the Cephalopoda are always feathered on both sides. Those of the 

 SiluriiicJiia have been the most thoroughly investigated. In Sepia, eaeh gill has 

 the shape of a slender cone, its "whole length being applied to the visceral dome in 

 the mantle cavity, in such a way that the ba,se is directed dorsally towards the ape.K 



of the visceral dome, and the point ventrally 

 towards the free edge of the mantle fold or 

 the mantle cleft (Fig. 80, p. 83). The 

 points of the two gills diverge. 



The two rows of flat triangular branchial 

 leaflets (Fig. 92) are carried by the two 

 branchial vessels, each leaflet being attached 

 by one end of its base to the branchial artery 

 and by the other to the branchial vein. In 

 the axis of the gill between the two vessels, 

 and also between the bases of the two rows 

 of leaflets, a channel is formed which com- 

 municates by a slit between each successive 

 pair of leaflets with the mantle cavity ; 

 through this canal the respiratory water 

 freely flows. The slits in this axial channel 

 are arranged alternately on each side, like 

 the leaflets between whose bases they lie. 

 The branchial vein forms the jiosterior sup- 

 port of the gill turned towards the mantle, 

 and the branchial artery the anterior support 

 turned towards the visceral dome. The 

 artery is united along its entire length with 

 the integument of the visceral dome by a 

 membrane of connective tissue. The an- 

 terior edge of each leaflet (that facing the 

 visceral dome) is connected with this mem- 

 brane, which may be called the gill-suspen- 

 sor, by means of another triangular mem- 

 brane. A special vein runs along the 

 posterior free edge of each leaflet, and enters 

 the general branchial vein at its base ; and 

 a special artery runs along the anterior edge, 

 i.e. along that edge of the leaflet which is 

 fastened to the suspensor. Each leaflet is wrinkled in suclua way that the folds 

 on the two surfaces alternate, each fold being creased in its turn. These two systems 

 of folds «;ross each other at right angles, and serve to increase the respiratory surface. 

 At the point where the suspensor of the gill passes into the integument of the 

 visceral dome, it contains a cellular body, which is traversed by a system of inter- 

 cellular blood-channels. This may perhaps be a blood-making gland. It receives 

 venous blood from branches of the principal branchial artery and of the special 

 arteries of the leaflets, and returns the same along two^veins which run back to the 

 base of the gill, there, with others, to open into the venous sinus of the renal organ ; 

 from this organ the blood passes for the second time along the branchial artery into 

 the gill. We thus find that not all the venous blood which is conducted by the 

 branchial artery towards the gills enters the leaflets for purposes of respiration ; part 



MO 



Fig. 92.— Diagram to illustrate tlie struc 

 ture of tlie gill of Sepia (after Joubin). I, 

 Branchial vein (containing arterial blood) ; •!, 

 brancliial canal ; 3, brancliialj artery (contain- 

 ing venons blood); 4, special branchial vein 

 (vas efferens) of each leaflet ; 5, special branchial 

 artery (vas alferens) of each leaflet ; r., suspensor 

 ' if the gill, which attaches tlie brancliial artery 

 (3) to the posterior integnment of tlie visceral 

 dome (12) ; 7, suspensor of eacli leaflet to tlie 

 general suspensor (6) ; S, one of tlie connecting 

 vessels between tlie branchial artery and tlie 

 "blood-making gland" (9), tlirougli which 

 \enous blood flows; 10, 11, vessels carrying 

 the venous blood which has passed through 

 the " blood-making " gland baclv to the venous 

 sinus at tlie base of the gill. The arrows in- 

 dicate the direction of tlie blood-stream. 



