GASTEROPODA 



557 



supplying the buccal cavity and a posterior which supplies the visceral 

 hump. The terminal branches of these arteries eventually communi- 

 cate with the general haemocoele (stippled in Fig. 381) and this 

 discharges into the cir cuius venosus leading to the lung and heart. 



The alimentary canal (Fig. 382) commences with the buccal mass. 

 On the roof of the mouth is a small transverse bar, the jaw, and in 

 conjunction with this works the radula, which is a strip of horny base- 

 ment membrane on which are fastened many rows of minute recurved 



ped.art 



haem. 



Fig. 381. 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 network 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. afferent 

 veins; ao. aorta; art.cap. arterial capillaries; au. auricle; buc. 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. 



teeth. It is formed in a ventral diverticulum of the buccal cavity called 

 the radula sac (Fig. 383) in which proliferating tissue is constantly 

 producing transverse rows of cells called odontoblasts, each of which 

 helps to form a tooth, and other cells which secrete the basement 

 membrane. The whole radula is pressed forward by the new growth 

 so that fresh surfaces are constantly coming into use as the old part 

 is worn away. The radula is supported by masses of tissue, resembling 

 cartilage, which also serves for the attachment of muscles, and the 

 whole forms the rounded organ which is the buccal mass. 



