538 



THE MICROSCOPE. 



strap, ■vvliicli bears a long series of transversely-disposed 

 teeth. Tlie ends of the strap are connected with muscles 

 attached to the upper and lower surface of the hinder ex- 

 tremities of the cartilaginous cushions ; and these muscles, 

 by their alternate contractions, cause the toothed strap to 

 work backwards and forwards over the end of the pulley 

 formed by its anterior end. The strap consequently acts 



Fig. 251. 



1, Palate of Bucdnum vndatum, o.c.mmon Whelk, seen under polarised light. 

 2, Palate of Doris tuberculata, Sea-slug. 



after the fashion of a chain-saw (rather of a rasp,) upon 

 any substance to which it is applied, and the resulting 

 wear and tear of its anterior teeth are made good by the 

 incessant development of new teeth in the secreting sac in 

 which the hinder end of the strap is lodged. Besides the 

 chain-saw-like motion of the strap, the odontophore may 

 be capable of a licking or scraping action as a whole. "^ 



In the constant growth of the band we observe the 

 development of new teeth. In some the teeth on the 

 extreme part of the band differ much, both in size and 

 form, from those in the median line : so much, that if at 

 any time one portion be separated from the other and 

 then examined, it might be supposed to belong to another 

 species. 



Since the investigations of Professor Loven, of Stock- 

 holm, into the lingual dentition of the glossophorous Mol- 

 lusca, various observers have studied the subject with 

 great advantage to our knowledge of the affinities of those 

 animals. Although the patterns or types of the lingual 

 membranes are, on the whole, remarkably constant, yet 



(1) Elements of Comi-arative Anatomy, p. 36. 



