202 GASTROPODA. 



formation of the next tooth of the same longitudinal row. The 

 number of teeth in a transverse row corresponds to the number of 

 groups of odontoblasts. The formation of the radula is, however, 

 not altogether completed by the processes just described, for the 

 upper epithelium yields a viscid fluid secretion which forms an 

 enamel-covering to the teeth. The gradual shifting forward of the 

 newly-formed teeth to replace those which are continually being 

 worn away in front, is brought about to a great extent by the 

 growth of the surrounding tissues, and is no doubt also caused by 

 the action of the muscles at the anterior part of the odontophore 



(ROSSLEE). 



The radula appears to form in other Molluscs that are provided 

 with it (Cephalopoda, Fig. 91 A, and Amphineura) in just the 

 same way as in Gastropods ; it will not, therefore, be necessary to 

 describe it in detail again. 



The salivary glands arise somewhat late as diverticula of that 

 part of the stomodaeum which lies in front of the radular sac. 



The enteron, in various Gastropods, arises to a certain extent in 

 a different way, as the accumulation of yolk or of a secondary 

 nutritive mass at various points of the gut frequently retards its 

 development and may even, where the mass is very voluminous, 

 strongly influence the manner of formation of the intestinal canal. 

 In many cases, however, the formation of the enteron takes a very 

 simple course, the invaginated entoderm-vesicle increasing in size by 

 the continuous division of its cells, fusing anteriorly with the stomo- 

 daeum and growing out posteriorly into a conical terminal section 

 which becomes connected with the ectoderm to form the anus. It 

 has already been explained that the posterior section of the enteron 

 may at first run straight back, but may later bend forward to the 

 right, and that this is connected with the acquisition of asymmetry. 

 The coils made by this section of the gut as it lengthens are not of 

 essential importance and need not therefore be specially described. 

 There are, however, other important alterations brought about by the 

 deposition 'of nutritive masses in the enteron. This process of deposi- 

 tion takes place in a very simple manner in Paludina (BiiTSCHLi). 

 The ventral part of the entoderm here becomes even at early stages 

 especially large through the increase in size of the cells and the 

 deposit in them of drops of secondary yolk (Figs. 57, 58 and 59, p. 

 137, etc.). This thickening of the wall of the enteron is evidently 

 due to the absorption of the surrounding albumen ; this albumen being 

 received especially into the ventral entoderm and deposited there. At 



