372 



COMPAEATIVE ANATOMY. 



parts of the body, and generally gives off a large branch to the foot ; 

 this sometimes looks as if it were a continuation of the chief trunk. 

 In addition to this, it gives off on its course branches which pass to 

 the stomach, salivary glands, and so on ; it either ends simply, or 

 after several ramifications, in the region of the pharynx. In the 

 Pteropoda it is more widely distributed, for in them the pedal branch 

 has the characters of a continuation of the chief trunk, and in the 

 head it divides into two large terminal branches which ramify lai-gely 

 in the fins. The visceral artery, which corresponds to the posterior 

 artery of the Lamellibranchiata, is but slightly broken up in the 

 Pteropoda and lower Gastropoda. Like the cephalic artery, it loses 

 itself in larger haemal spaces. In most of the Prosobranchiata it is 

 greatly developed, and much broken up {ap) ; this is the case also in 

 many of the Tectibranchiata (Pleurobranchus), but in others it is 

 replaced by several smaller arteries (Aplysia). The visceral arteries 

 of the Nudibranchiata are branches of the principal trunk (Doris). 



The afferent vessels differ according to the number, form, and 

 disposition of the respiratory organs. In many of the Nudibran- 

 chiata the blood is collected from the body-cavity near the auricle. 

 In others, which have distinct respiratory organs, there are definite 

 canals, or even vessels with special walls, which convey the blood 

 from the venous passages to the respiratory 

 organs. "When these are most simple in character, 

 the blood passes to the auricle without going- 

 through branchial veins. This is the case, too, 

 in many of the Pteropoda. When the gills are 

 more highly developed the returning blood is 

 collected into special venous trunks, which open, 

 either separately or together, into the auricle. 

 These branchial veins are always so arranged as 

 to be adapted to the size as well as to the posi- 

 tion of the branchiae. In many of the Nudi- 

 branchiata (^olidia, Scyllsea, Tritonia), large 

 vessels pass off from the gills and unite together 

 to form larger trunks, which give rise to a median 

 or two lateral branchial venous trunks, connected 

 with the auricle of the heart. When the bran- 

 chi^ are scattered over a larger portion of the 

 surface of the body, this afferent system of bran- 

 chial vessels is well developed, but when they are 

 limited in extent it is reduced in size (Doris, 

 Polycera). Tritonia is an example of the former 

 arrangement (Fig. 198), for in it two lateral bran- 

 chial venous trunks {ss) pass by means of a trans- 

 verse trunk to the heart. The transverse canal 

 forms a kind of double auricle {a), although indeed it has only one 

 opening into the ventricle {v). 



The paths by which the blood goes to the branchi^ are always 

 more or less lacunar. In many of the Opisthobranchiata the blood 



Fig. 198. Part of 

 the circulatory or- 

 gans of Tritonia. 

 s Venous sinuses 

 laid open. The wall 

 is perforated by 

 the openings of 

 the branchial veins. 

 V Ventricle. 



