246 



ZOOLOGY. 



assist in respiration. (For figures of the gill structure in the 

 clam see Parker and Haswell's Text-book of Zoology, Vol. I, 

 Fig. 5 2 9-) 



286. Circulation. There is usually a well-developed circu- 

 lation of the blood, but a portion of it occurs through irregular 

 spaces devoid of proper walls. The organs consist of a con- 

 tractile heart usually with undivided ventricle and a single 

 auricle (gasteropods), or one pair of auricles (lamellibranchs, 



tissues 



tissues 



FIG. in. Diagram showing the general course of the circulation in mollusks. Com- 

 pare with Fig. no, which shows the organs more nearly in their relative position. 



Questions on the figure. Why does the blood which passes to the 

 mantle not need to pass to the gills before returning to the heart? What 

 happens to the blood in each of the regions named in the diagram? 



squid), or two pairs (Nautilus). Definite arteries pass both 

 forward and backward from the ventricle. The blood passes 

 from the ventricle to the tissues of the body, whence it gathers 

 into venous spaces and passes into the kidneys and the gills. 

 From the gills it finds its way to the auricles. In lamelli- 

 branchs the bloojd which goes from the ventricle to the mantle 

 returns directly to the auricle. In some Cephalopods there 

 are branchial hearts near the gills to assist the return of the 

 blood to the heart. The accompanying diagrams (Figs, no, 

 m) will help the student follow the main facts of the circula- 

 tion. In lamellibranchs the ventricle often surrounds the in- 



