TYPE MOLLUSC A. 



279 



of the blood with those of the water contained in the mantle- 

 cavity being thus readily effected, a renewal of the water con- 

 stantly taking place in consequence of the action of the ecto- 

 dermal cilia. The thin-walled niautle-fold is, however, a very 

 efficient adjunct to the branchiae in respiration, the spaces 

 within the fold being portions of the lacuuar ccelom and con- 

 seqiieutly containing blood ; indeed in some cases the mantle 

 assumes completely the respiratory function, the cteuidia 

 becoming rudimentary. 



The digestive tract is a usually more or less coiled or con 

 voluted tube in which various regions may be distinguished. 

 In a few forms, characterized either by the slight develop- 

 ment of the mantle or its development as two lateral folds, 

 the anus is terminal in position, but when an extensive cir- 

 cular mantle-fold is developed the intestine bends upon itself 

 and opens upon the side of the body, more or less anteriorly, 

 into the mantle-cavity. Immediately behind the mouth chi- 

 tinous teeth (Fig. 123, Jij ) are usually developed in the wall of 



st 



FIG. 123. BUCCAL MASS AND RADULA OF Helix (after HOWES). 



ce = cerebral ganglion. re = radular cartilage. 



lij = horny jaw. rd = radula. 



im = intrinsic muscles. rd' = radular sac. 



= pedal gland. st opening of salivary gland. 



the pharynx, and behind these a large muscular thickening 

 generally occurs, the buccal mass, in connection with which 

 is developed a characteristic Molluscan structure, the lingual 

 ribbon or radula (rd). The sides and floor of the phaiynx in 

 this region are largely thickened by the development in them 

 of muscular tissue (im). The thickening of the floor is usually 

 so extensive as to project into the pharyngeal cavity, forming 



