932 



MOLLUSCA AND BBACHIOPODA 



part most developed, and contains a number of rows of conical teeth, 

 standing almost perpendicularly, like those of a harrow (fig. 708). 



Many other varieties might be described did space permit ; but 

 we must be content with adding that the form and arrangement of 

 the teeth of these ' palates ' afford characters of great value in classi- 

 fication, as was first pointed out by Professor Loven (of Stockholm) 

 in 1847, and has been since very strongly urged by Dr. J. E. Gray, 

 who considers that the structure of these organs is one of the best 

 guides to the natural affinities of the species, genera, and families of 

 this group, since any important alteration in the form or position of 

 the teeth must be accompanied by some corresponding peculiarity in 

 the habits and food of the animal. 1 Hence a systematic examination 

 and delineation of the structure and arrangement of these organs, by 

 the aid of the microscope and camera lucida, would be of the greatest 

 service to this department of natural history. The short thick tube 



of Limax and other terrestrial 

 Gastropods appears adapted for 

 the trituration of the food pre- 

 viously to its passing into the 

 oesophagus ; for in these animals 

 we find the roof of the mouth 

 furnished with a large strong 

 horny plate, against which the 

 flat end of the tongue can work. 

 On the other hand, the flattened 

 portion of the palate of Bucci- 

 num (whelk) and its allies is 

 used by these animals as a file, 

 with which they bore holes 

 through the shells of the molluscs 

 that serve as their prey ; this 



they are enabled to effect by everting that part of the proboscis- 

 shaped mouth whose floor is formed by the flattened part of the 

 tube, which is thus brought to the exterior, and by giving a kind of 

 sawing motion to the organ by means of the alternate action of 

 two pairs of muscles a protractor and a retractor which put 

 forth and draw back a pair of cartilages whereon the tongue is 

 supported, and also elevate and depress its teeth. The use of the 

 long blind tubular part of the palate in these Gastropods is that 

 of a ' cavity of reserve,' from which a new toothed surface may be 

 continually supplied as the old one is worn away somewhat as the 

 front teeth of the rodents are constantly being regenerated from the 

 surface of the pulps which occupy their hollow conical bases as fast 

 as they are rubbed down at their edges, or as a nail is constantly 

 being worn away at its free end, and fashioned anew in its 

 1 bed.' 



The preparation of these palates for the microscope can, of course, 

 be only accomplished by carefully dissecting them from their attach- 

 ments within the head ; and it will be also necessary to remove the 

 membrane that forms the sheath of the tube, when this is thick 

 1 Ann. Nat, Hist. ser. ii. vol. x. 1852, p. 413. 



FIG. 708. Palate of Doris tuberculata. 



