74 Evolution and Distribution of Fishes 



nected with it by the stylet zone. Its inner surface epi- 

 thelium is highly glandular. 



Hubrecht considered that in transition from nemerteans 

 to fishes, the entire proboscis became greatly shortened 

 and condensed, and during embryological development sep- 

 arated from the front rim of the enclosing sheath or rhyn- 

 chocoel, then became bent upward posteriorly, and uniting 

 with a downgrowth of the brain — the infundibulum — be- 

 came the important glandular tube that is known as the hypo- 

 physis cerebri or pituitary body {-fi '35^-3 SS) • ^^^ the 

 writer has suggested what seems a more complete explan- 

 ation of the evolutionary changes involved (7:420). For 

 since the proboscis in freshwater and land nemerteans 

 is an ingrowth of the posterior buccal cavity, the anterior 

 muscular eversible part, as It underwent steady conden- 

 sation and shortening in the evolution of primitive fishes, 

 might gradually fuse with and spread over the buccal 

 cavity, or stomodoeum. Thus would arise the muscular 

 buccal area of vertebrates, and specially the powerful suc- 

 torial disc that is typical of cyclostomes, sturgeons, larval 

 gar-pike, of some teleosts, also of larval urodele amphibians 

 and even of mammals. A tendency to the formation of 

 stylets, and ultimately from them of buccal teeth, might 

 spread over the entire area. This would account for their 

 abundance in Petvomyzon, as well as In some of the true 

 fishes. 



The highly muscular mid-proboscis may, on Its ventro- 

 lateral sides have condensed and grown forward In simple 

 or in bllobed form as the tongue of cyclostomes, elasmo- 

 branchs, dipnoans and crossopterygian fishes; while the 

 stylets may have been replaced by the strong buccal teeth 

 of Petromyzon or the ventral plates of teeth in higher 

 fishes. Such a view is decidedly favored also by Dean's 

 statement {42: 57) that In the cyclostome Bdellostoma the 

 tongue, which is bllobed and "studded with rows of rasp- 

 like teeth, may be greatly everted, and then drawn in by 

 stout tongue muscles." The nerve threads that passed 

 to and innervated this anterior portion seem grad- 

 ually to have separated from their posterior roots, and 



