GASTEROPODA 483 



number of folds covered by cells containing uric acid. The ureter is 

 a thin-walled tube which runs along the right border of the mantle 

 cavity parallel to the rectum and opens just behind the pneumo- 

 stome and above the anus. Here again is a difference from the marine 

 gasteropods in which the anus and kidney aperture discharge inside 

 the mantle cavity, faeces and urine being swept away by the respira- 

 tory current. The pericardium and the kidney represent the coelom 

 in the snail and, as is usual in Mollusca, their common derivation is 



ped.art 



haem. 



Fig. 357. Helix pomatia. Diagram of the circulation and haemocoelic spaces. 

 The pulmonary veins, ventricle and arteries are shown in black; the veins 

 and haemocoelic spaces are indicated by stippling. Only a few of the arteries 

 are shown and a small portion of the arterial capillary netw^ork in the posterior 

 part of the foot. The course of the columella muscle and its branches is 

 indicated. The direction of the blood flow is shown by arrows, aff.v. aflferent 

 veins; ao. aorta; art.cap. arterial capillaries; au. auricle; hue. buccal mass; 

 col. columella, col.m. columella muscle ; cr. crop; c.v. circulus venosus; haem. 

 haemocoelic spaces; k. kidney; n.col. nerve collar; ped.art. pedal artery; 

 pul.v. pulmonary veins; ten. tentacles ; ven. ventricle. 



shown by the connection of the cavities by the renopericar dial canal. The 

 coelom, though thus represented, does not constitute the perivisceral 

 cavity. On cutting the floor of the mantle cavity and continuing 

 the cut forward towards the mouth a large body cavity is revealed 

 which contains the anterior part of the alimentary canal and the greater 

 part of the reproductive organs. This is a haemocoele almost as well 

 developed as that of arthropods. Its connection with the rest of the 

 blood system and the general course of the circulation may be briefly 



31-2 



