Chap, iv.] ENTERON OF MOLLUSCA. 137 



and aids the radula in its work of trituration, just as 

 the hard pad which takes the place of the upper in- 

 cisors of the sheep serves as a resistent structure 

 against which the lower incisors may bite. 



At the sides of the anterior portion of the digestive 

 tract glands of various forms are ordinarily found ; 

 these are known as salivary glands ; but the inappro- 

 priateness of the name is not only obvious from the 

 observed fact that in the slug the secretion of these 

 glands has no influence on starch, but is made the 

 more striking so soon as we know that, in several 

 genera, the secretion of these glands contains a com- 

 paratively large amount (nearly three per cent, in 

 Dolium) of free sulphuric, and a smaller quantity of 

 hydrochloric, acid. Further, we have to note that 

 these buccal glands are found in marine as well as in 

 terrestrial forms, whereas among the vertebrata the 

 salivary glands are only well developed in terrestrial 

 forms. 



The intestine is considerably coiled ; the resopha- 

 geal portion is sometimes produced into a " crop," as 

 in Lymnseus or Octopus ; the succeeding portion may 

 be simple, and have its walls thin or muscular, or it 

 may be broken up into several chambers ; in Scyllsea 

 it is armed internally with horny cutting blades, and 

 in Aplysia with blunt horny spines, behind which is 

 an armature of sharp hooks. It is only behind such 

 gizzard-like enlargements that the digestive ferments 

 are secreted. The anal orifice is, in those Cephalo- 

 phora that have lost their original bilateral symmetry, 

 brought far forwards, and situated near the mouth, 

 or is placed at the side of the body. Csecal pouches 

 or tubes are developed in various ways along the tract 

 of the intestine, and some of them become charged 

 with dark-coloured cells, and have been regarded as 

 forming a " liver ; "' there is, however, no reason for 

 associating with these structures the functions of the 



