Evolution of Animals 457 



"the mucous membrane" of the sacs "is raised into radial folds" 

 (138:^00). 



Much speculation might be indulged in as to the pseudo- 

 nares of many Apoda. Shortly it might be suggested that 

 these are the paired frontal organs of some nemerteans still 

 persisting in reduced state, or even are the paired functional 

 organ of Bdellostoma. With good show of reason it might 

 thus be claimed that they become the paired nasal sacs of 

 higher craniates. Carefully prepared embryological and adult 

 histological material can alone throw proper light on the ques- 

 tion. 



As to the histology of the organ in the three groups, Burger's 

 description of the frontal organ in nemerteans is unfortunately 

 scant. His figures strongly suggest that it is lined by cells 

 of two kinds, in one of which the cells are elongate and bear 

 a fine tactile process. These two kinds seem to correspond 

 to Schultze's olfactory cells and epithelial cells, to the former 

 of which olfactory nerve-threads pass according to Blanc. 

 In the Cyclostomata and Apoda the structural details are 

 similar to the last, but in Mammalia the epithelial cells have 

 been crowded out by the olfactories which now wholly line 

 the cavity (137: 513). ' 



The eye and ear need not concern us further than as already 

 discussed (p. 429). 



The ahmentary canal presents points of comparative in- 

 terest. 



Wiedersheim says (138: 250): "The glands of the mouth, 

 like those of the orbit and integument, appear first in terres- 

 trial vertebrates, that is, from amphibians onwards. They 

 have the function of keeping moist the mucous membrane 

 which comes into contact with the outer air. From being 

 at first almost entirely unspecialized and giving rise simply 

 to a slimy fluid, they become difterentiated later into struc- 

 tures the secretions of which take on a very important func- 

 tion in relation to digestion." 



In contrast to the above, Lonnberg (11^7: 316) says of the 

 lower epithelial layer of the oral cavity in Myxine: "The 

 basal cells are small-polygonal, between them appear larger 

 mucilage cells which are quite similar to the mucilage cells" 

 in the upper epidermis. (See also Retzius, IJ^.8.) But further 

 Haack for Petromyzon has carefully described paired salivary 

 glands which lie directly under the tongue and open by special 

 papillae. Of particular interest is it to note that these are 



15* 



