GASTEROPODA 



559 



382 A). The secretory cells produce the brown fluid found in the crop ; 

 this contains a ferment which dissolves the cellulose of plant cell walls 

 and liberates the protoplasmic contents, no portion of which is di- 

 gested in the crop or stomach. But these contents in the form of small 

 granules are actually introduced into the alveoli of the liver and there 

 taken up and digested by the resorptive cells which possess intra- 



odp.'^- 



Fig. 383. Vertical longitudinal section through head of iiTe/iA;. After Meisen- 

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

 j. jaw ; M. mouth ; m.r. muscles of radula ; odp, odontophore (radula sac) ; 

 oe. oesophagus ; p.g. pedal ganglion ; rad. radula ; v.g. visceral ganglion. 



odh. 



Fig. 384. 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, b?n., 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 fonvard in the direction of the arrow and themselves 

 reinforce the basal epithelium. 



cellular proteolytic enzymes.^ A combination of extra- and intra- 

 cellular digestion is highly characteristic of Mollusca, but in the 

 possession of a cellulose-dissolving ferment Helix stands almost alone 



^ In carnivorous gasteropods digestion follows a different course. The 

 glands of the alimentary canal secrete proteolytic enzymes and the digestion 

 of protein takes place in the stomach and not in the cells of the digestive gland 

 (see Murex, p. 566). 



