XII 



PHYLUM MOLLUSCA 



745 



of veins. A large median vein, the vena cava (v. cav.), runs from 

 the head to the neighbourhood of the rectum, in front of which 

 it bifurcates to form the left and right afferent branchial veins 

 (/. off. br. v. } r. aff. br. v.), each running through the cavity of 

 the corresponding renal organ to the base of the gill, where it is 

 joined by veins from the aboral region. At the base of the gill 

 the afferent branchial vein becomes dilated to form a contractile 

 sac the branchial heart (r. br. Tit.) appended to which is a rounded 

 body of a glandular character the appendage of the branchial 

 heart, representing the pericardial glands 

 of the Pelecypoda. The afferent branchial 

 vein runs through the axis of the branchia, 

 giving off branches as it goes. The 

 blood is carried back to the ventricle 

 on either side by a dilated contractile 

 vessel, the auricle or efferent branchial 

 vein (1. aur. t r. aur.). 



The ccelome (Fig. 680) is a pouch of 

 considerable size, divided by a constric- 

 tion into oral and aboral parts. The 

 former is the pericardium, or cavity in 

 which the heart is lodged ; it gives off 

 a pair of diverticula, right and left, each 

 lodging the corresponding branchial 

 heart, and communicates by a pair of 

 apertures with the cavities of the kidneys 

 or renal sacs. The aboral part of the 

 ccelome forms the capsule (gonoccele) 

 which encloses the ovary or testis. 



The paired, plume-shaped ctenidium 

 lies parallel with the long axis of the 

 body. It is attached throughout the 

 greater part of its length to the wall of 

 the mantle-cavity by a thin muscular 

 fold, and consists of numerous pairs of 

 delicate lamellae, the surface of which is 

 increased by the presence of a complex 

 system of foldings. Internally the 

 lamellae are not completely in contact, an axial canal being left 

 through which the water penetrates freely to all parts of the gill. 

 The blood carried to the gill by the afferent branchial vessel 

 passes in a System of minute branches through the lamellae, and is 

 gathered up again into vessels which open into the main efferent 

 vessel leading to the auricle. 



Nervous System. Though parts homologous with those 

 of Triton are recognisable in the nervous system of Sepia, their pro- 

 portions and arrangement indicate a higher grade of organisation. 



FIQ. 670. Sepia officinalis, 



enteric canal, a. anus ; b. d. duct 

 of one of the portions of the diges- 

 tive gland ; b. m. buccal mass ; c. 

 caecum ; i. ink-sac ; t. d. ink-duct ; 

 j. jaws ; 1. I. digestive gland ; a?, 

 oesophagus ; p. pancreatic appen- 

 dages ; r. rectum ; s. g. salivary 

 glands ; st. stomach. (From the 

 Cambridge Natural History.) 



