ANATOMY AND PHYSIOLOGY OF A SNAIL. 



33 



On cutting open the pedal sinus the openings of these transition 

 vessels into this sinus can be observed. There are four venous 

 sinuses, two pedal sinuses, a visceral or marginal sinus, and a 

 pulmonary sinus or circulus venosus pulmoms. The pedal sinuses 

 run along the foot, one on each side ; the marginal sinus runs 



Fig. 9. Nervous and circulatory systems of Pahidiua vivipara (after Leydig). 

 F, tentacle ; Oe, oesophagus ; Cg, supra-oesophageal or cerebral ganglion, with eye ; 

 Pg, pedal ganglion with adjacent otocyst ; Vg, visceral ganglion ; Phg, infra-ceso- 

 phageal or pharyngeal ganglion ; A, auricle of h<*art ; Ve, ventricle ; Aa, posterior 

 aofta ; Ac, anterior aorta ; V t veins ; Ve, afferent vein ; Br> gill. 



from the apex of the spire to end in the circulus venosus pulmonis, 

 a very large sinus which surrounds the base of the mantle cavity. 

 From the last named sinus numerous afferent vessels the afferent 

 pulmonary veins arise to form a plexus on the wall of the mantle 

 cavity, and from these efferent vessels the efferent pulmonary 

 veins carry the blood to the nephridium, where they break up 

 into a plexus the renal plexus the venules of which join together 

 to form a renal vein, which unites with a vein the pulmonary 

 vein formed by the coalescence of a large number of efferent 

 pulmonary veins. The pulmonary vein enters the auricle ; it has 

 been seen by 'Nalepa to pulsate rhythmically in Zonites algirus. 

 The marine snail, Haliotis, has its heart somewhat similar to that 

 organ in Anodonta cygnea. Its auricle is divided into two cavities, 

 and its ventricle is pierced by the rectum. In Fissurella and in 



