VASCULAE SYSTEM OF MOLLUSCA. 



371 



from the gills is received into the auricles, all of the blood finally 

 returns, by various ways, to the ventricle. 



The character of the circulation in the glands of Bojanus is 

 worthy of remark. These excretory organs are placed in the 

 way of the blood which is going to the gills, and which, there- 

 fore, is venous ; and herein we get a kind of primitive portal 

 circulation. 



286. 



In the Gastropoda, the heart, which is here also enclosed in a 

 pericardium, is often provided with two lateral auricles (Haliotis, 

 Fissurella, Nerita). Trochus also has two auricles, but the left one 



Fig. 197. Organisation of Paludina vivipara. c Head, t Tentacles, p Foot. 

 op Operculum, o Eye. a Auditory organ, n Cerebral ganglion, n' Pedal ganglion. 

 n" Branchial ganglion, n'" Buccal ganglion. (The commissure from the cerebi'al 

 ganglion is not figured.) ph Pharynx, ce (Esophagus, br Branchiae, r Benal organs. 

 s Venous sinus. Sv Venous sinus at the base of the branchiae. / Branchial artery. 

 at Auricle of the heart, v Ventricle, ap Posterior artery (visceral artery), aa Anterior 



artery (after Ley dig). 



is somewhat reduced ; in the rest of the Gastropoda, as also in the 

 Pteropoda, there is but one auricle (Fig. 197, at). The dorsal position 

 of the heart is modified by the asymmetrical development of the 

 visceral sac ; it is always placed close to the respiratory organs, and 

 the thin-walled auricle is always directed towards them. It is 

 placed in front of them in the Prosobranchiata, and behind them in 

 the Opisthobranchiata. It has in many Gastropoda the same rela- 

 tion to the rectum as it has in the Lamellibranchiata (Turbo, 

 Nerita, Neritina); in some, indeed, the ventricle is divided (Haliotis, 

 Fissurella, Emarginula). 



An aorta arises from the ventricle and gives off a visceral artery 

 which passes backwards (fl£>), while the trunk is continued on as an 

 aorta cephalica (aa). This latter passes directly to the anterior 



2 b 2 



