MOLLUSCA— RESPIRATORY ORGANS 



the base of the branchial groove (mantle cavity). The axis hei'e takes the shape of 

 a thin septum. At each side, on the broader surface of the septum, extending fiom 



Fio. 82.— Structure of the ctenidium of a Chiton (after B. Haller). A, Single ctenidium with 

 its double row of liranchial leaflets. B, Transverse section of the gill along the line a-h in Fig. A. 

 1, Narrow blood -sinus in the branchial leaflet; 2, septum in its axis; 3, longitudinal muscle; 4, 

 afferent brancliial vessel ; 5, efferent branchial vessel ; 6, nerves ; 7, long cilia on the branchial 

 axis. C, 2 pairs of branchial leaflets cut througli at right angles to their surfaces, along tlie line c-f 

 in Fig. B. 1, t^ame as in Fig. B; S, space between the consecutive branchial leaflets. D, Longi- 

 tudinal section of the ctenidium somewhat laterally to the axis, and parallel to its septum, along 

 the line c-d in Fig. A. This section is part of a transverse section of the body. Lettering as in 

 Figs. B and C. In addition : 9, olfactory ridge of the branchial epithelium ; 10, general afferent 

 branchial vessel ; 11, general efferent branchial vessel ; 12, pleuro-visceral strand of tlie nervous 

 system. Tlie branchial epithelium is everywhere indicated by a thick black line. 



base to tip, there is one row of smooth, delicate branchial leaflets. In outline they 

 are more or less semicircular, and stand crowded together in great numbers almost 

 like the leaves of a book. The entire surface of the branchial epithelium is 

 ciliated ; on the axial epitlie- 



Hum, the cilia are remarkably A B ^ 



long. On that side of the axis 

 which is turned towards the 

 foot, a blood-vessel runs from 

 base to tip, conducting venous 

 blood to the gill (afferent 

 branchial vessel). On the op- 

 posite side, which faces the 

 mantle, another vessel, the 

 branchial vein, runs from the 

 tip to the base of the gill, and 

 cai'ries the blood, which has 

 become arterial by respiration, 

 to the general branchial vein, 

 and through it to the auricle. 

 These vessels have no special 



Fid. s:'..— Diagrams illustrating the arrangement of the 

 gills in the Chitonidse. «i, Mantle ; o, mouth ; k, snout ; /, 

 foot ; ct, ctenidia ; n, anus. 



endothelial walls, but are surrounded by circular muscle fibres. The branchial vein 

 is accompanied by a powerful longitudinal muscle. At the base of each branchial 

 leaflet, the blood flows out of the branchial artery through an aperture into the 

 narrow cavity of the leaflet, and jjasses through a similar aperture on the opposite 

 side of the axis to enter the branchial vein. Nerves are supi)lied to the ctenidium 

 from the pleuro-visceral nerve which runs close to its base. 



