TYPE MOLLUSOA. 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 mantle-fold is, however, a very 

 efficient adjunct to the branchiae in respiration, the spaces 

 within the fold being portions of the lacunar coelom and con- 

 sequently containing blood ; indeed in some cases the mantle 

 assumes completely the respiratory function, the ctenidia 

 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, hj) are usually developed in the wall of 



8t 



pgl rd 



Fig. 133. — Buccal Mass and Eadula of Helix (after Howes). 



ce = cerebral ganglion. re = radular cartilage. 



?tj = horny jaw. rd = radula. 



im = intrinsic muscles. rd' = radular sac. 



pgl = pedal gland. st = opening of salivary gland. 



the pharynx, and behind these a large muscular thickening 

 generally occiirs, the buccal mass, in connection with which 

 is developed a characteristic Molluscan structure, the lingual 

 ribbon or radvla (rd). The sides and floor of the pharynx 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 



