64 Evolution and Distribution of Fishes 



are so typical of the main groups of fishes. The latter, by 

 increasing specialization in development, and condensing 

 modification of the front part of the proboscis, would form 

 the horny to calcareous teeth of the buccal region in cyclos- 

 tomes and higher fishes. 



So from some primitively related genera of metanemer- 

 teans, that had all early developed proboscidial stylet 

 teeth, which later became buccal or pharyngeal teeth, we 

 would trace two divergent lines of evolution. One of these, 

 remaining soft-bodied in its epidermis, pursued one path- 

 way of progress that led to the cyclostomes and later to 

 primitive ancestors of the caecilian amphibians. The other, 

 by secretion of horny and calcareous tubercles, led up to 

 Thelodus, Cladoselache and still higher types that made 

 up the main line of piscine development. But that the 

 calcified tubercles, the stylet teeth, and the buccal teeth 

 represent practically identical formations has been already 

 emphasized by Bridge (5(5:247-48) and others. 



IV. Sense-organs of the Head. 



These are (a) the frontal sense-organ of nemerteans 

 which we have correlated (/: 428) with the olfactory sense 

 organ of fishes; (b) the eyes or optic sense-organs; and (c) 

 the auditory sense-organs. 



(a) The frontal or olfactory sense-organ. In metane- 

 merteans, at the front end of the head, a small orifice opens 

 into a saccular depression that is lined with ciliated cells, 

 and is surrounded internally by an investment of muscular 

 tissue. The organ is innervated by a branch from the 

 anterior part of the dorsal ganglion. Above this frontal 

 organ lie clusters of glandular cells — the so-called "head- 

 glands" — and often the orifices of these open into the organ. 

 Burger has named this the taste organ. ' In Cerebratuhis 

 three such organs — one median and two lateral paired 

 ones — occur and show similar structure. From their posi- 

 tion, structure, early embryonic origin (<5:373), nerve 

 supply, and relation to the head-glands above, the writer 

 views such as an unpaired or paired {Cerebratuhis) ol- 

 factory organ. In Amphioxiis the olfactory organ is like- 

 wise a single saccular depression lined with ciliated cells. 



