476 HISTORY OF THE HUMAN BODY 



similar to the others, but longer, and thus greatly resemble 

 the terminal organs of the lateral line system. At their free 

 ends the sensory cells usually possess one or more modified 

 flagella, which project into a small space, that is formed about 

 them by the supporting cells, and communicates with the ex- 

 terior through a small opening. In mammals the taste-buds 

 are associated together in groups in connection with several 

 sorts of papillae, especially the circumvallate, and the foliate, 

 the latter not occurring in Man. 



The sense of smell is located in a pair of ectodermic cavities, 

 situated anterior to the eyes, thus forming the most anterior 

 of the sense organs. They are thus in the most favored po- 

 sition for organs of sense, and although the data are too in- 

 sufficient for theories, this fact suggest^ that they were the 

 earliest to develop and that the primaeval habitat was either in 

 mud or in the deep sea where the olfactory sense was of pri- 

 mary importance. Amphioxus gives no clew to this, for here 

 the sense of smell is located in a median ciliated pit at the 

 anterior end and pushed a little to the left side by the develop- 

 ment of the median fin. The early stages of the cyclostomes 

 furnish much material for speculation, but unfortunately there 

 is no certainty felt as yet concerning the meaning of the details 

 presented. Here (Fig. 129) there appear at the anterior 

 end two median invaginations, the more posterior of which is 

 the cavity of the mouth (stomatod&um). The anterior de- 

 pression is that of a median nasal cavity, which would suggest 

 a primitive condition and possibly a kinship with the ciliated 

 pit of Amphioxus were it not for the fact that it is supplied by 

 two olfactory nerves from as many olfactory lobes, showing 

 that the single or monorrhine condition has here been secondar- 

 ily attained from a previous paired (amphirrhine) one. From 

 the posterior wall of this depression there develops a tubular 

 process, which, in Myxine, connects ultimately with the 

 pharynx and thus forms a direct communication between nose 

 and throat, but in the other cyclostomes ends blindly and soon 

 disappears. This passage is of interest as a prophecy of the 

 similar connection to develop later in air-breathing vertebrates, 



