280 INVERTEBRATE MORPHOLOGY. 



the so-called tongue, and in addition to the muscular tissue 

 two or more pieces of cartilage, the radular cartilages (re), are 

 frequently found in it. Covering the tongue is a stout chi- 

 tinous membrane, the basal membrane, which bears upon 

 its surface a usually enormous number of chitinous teeth 

 arranged in transverse rows, so that the basal membrane 

 and the teeth, together constituting the radula (rd), recall 

 somewhat the appearance of a flat tile. Behind the tongue 

 the floor of the pharynx is produced downwards and back- 

 wards into a pouch, the radula-sac (rd'), sometimes of con- 

 siderable length, into which the radula is continued on 

 the ventral wall, the cells (odontoblasts) which form the teeth 

 as a cuticular secretion, lying at the bottom of the sac. The 

 tongue, with its radula, can be protruded to a greater or less 

 extent from the mouth by special protractor muscles, and its 

 intrinsic muscles serve to give it a slow licking movement, 

 whereby the radula acts as in the manner of a file or rasp 

 upon the object with which it is in contact. Owing to this 

 action the radula is continually being worn away at its anterior 

 end, but is also continually being pushed forward upon the 

 tongue by the addition of new teeth to its posterior portion 

 at the base of the radula-sac. 



In connection with the digestive tract various glands are 

 usually present, of which the most constant are the salivary 

 glands and the " digestive glands." The former open into 

 the pharynx and in some cases reach extensive development ; 

 their function for the most part is but little understood, but 

 in some predaceous Gasteropods their secretion has been 

 found to contain a considerable amount of free sulphuric acid 

 which probably serves to soften the calcareous shell of 

 Echinoderrns and other Mollusca which serve these forms as 

 food. The " digestive glands " open into a dilated portion of 

 the intestine, usually termed the stomach, and are usually 

 paired, voluminous, much-branched tubular glands w r hose 

 function is indicated by the name applied to them. They 

 seem to be the physiological representatives of the pancreas 

 of the Vertebrata, and to secrete digestive ferments which 

 are brought into contact with the food in the stomach. 



The nervous sj'stem of the Mollusca (Fig. 124), in accord- 



