MOLLUSCA. 



many branches to the head and foot ; while the other passes dorsal- 

 wards to the viscera (fig. 515, Aa, Ac). The arteries terminate by 

 opening into blood spaces of the body cavity without special walls, 

 from which the blood passes either through the branchial (pul- 

 monary) arteries, or directly, without traversing intermediate vessels 

 (Heteropoda and many Nudibranchiata), to the respiratory organs, 

 whence it is returned through branchial (pulmonary) veins to the 

 auricle. The arrangements described as obtaining in the LamelU- 

 branchiata, by which water is able to enter the blood spaces and 

 dilute the blood, are said to occur also in Gastropoda. 

 In only a small number of Gastropods is respiration effected 



FIG. 615. Nervous system and circulatory organs of Paludina vivipara (after Leydig). F, 

 tentacle ; Oe, oesophagus ; Cg, cerebral ganglion with eye ; Pg, pedal ganglion with 

 adjacent otocyst ; Vg, visceral ganglion; Pkg, pharyngeal ganglion; A, auricle of 

 heart; Ve, ventricle; Aa, abdominal aorta; Ac, cephalic aorta; F, veins; Fc, afferent 

 vein ; Er, gill. 



exclusively through the general integument. By far the greater 

 number breathe through gills, and many through lungs ; a few com- 

 bine branchial and pulmonary respiration. The gills are usually 

 foliaceous or pennate cutaneous appendages, which are generally 

 placed between the mantle and foot and enclosed by the mantle 

 fold; in rare cases they are exposed and placed on the dorsal 

 surface. The mantle cavity is therefore at the same time the 

 respiratory cavity. 



The primitive arrangement of the gills appears to be that found 

 in the Zeugobranchiata, in which there are two, one on each 

 side; but, usually an asymmetrical development takes place, and 



