GASTEROPODA 485 



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. 



v.y. p.g. m.r. 



Fig- 359- Vertical longitudinal section through head of Helix. After Meisen- 

 heimer. cart.r. cartilaginous support of radula; ce.g. cerebral ganglion; 

 y. jaw; M. month.', m.r. muscles of radula; odp. odontophore (radula sac); 

 oe. oesophagus ; /j.gf. pedal ganglion; rad. radula; v.g. visceral ganglion. 



Fig. 360. Vertical longitudinal section through the radula sac of Helix 

 pomatia. After Meisenheimer. odb. four rows of odontoblasts secreting a 

 tooth, to.; a. the most anterior row of odontoblasts which, together with the 

 basal epithelium, i.ep., of the radula sac, secrete the basal membrane, bm., to 

 which the teeth, to., are attached. As the odontoblasts complete the secretion 

 of a tooth they are succeeded by fresh cells from the epithelium of the radula 

 sac, s.ep., pressing forward in the direction of the arrow and themselves 

 reinforce the basal epithelium. 



The buccal cavity is succeeded by the oesophagus, which widens 

 out into the crop, which in life contains a brown liquid secreted by the 

 "liver". On the side of the crop are the branching white salivary 

 glands, which empty their secretion by two ducts running forward into 

 the buccal cavity. The secretion is partly mucus, partly digestive fluid 

 containing an enzyme acting on starch. The crop is succeeded by the 

 stomach; this is imbedded in the digestive gland (liver), which 



