458 Causes and Course of Organic Evolution 



absent in the larval Petromyzon, and only appear during the 

 transformation stage as solid cell rows, which later form a 

 lumen. They seem to be absent in Myxine. Mucous and 

 salivary glands are both present therefore in some cyclostomes. 

 The possible relation of these to the buccal glands described 

 and figured by Burger may be noted. 



In the Apoda "oral glands are abundant," these being goblet- 

 shaped with large nuclei. Similar glands occur in Urodela, 

 but beside these a median intermaxillary gland occurs in the 

 area of the nasal septum. 



While the tongue in ordinary fishes is merely a slight fold 

 of mucous membrane, in the cyclostomes it is a large ventral 

 mass of oral tissue, broadly attached to the floor of the mouth, 

 and which in Myxine is covered with teeth so as to form a 

 rasping apparatus of great efficiency, that is worked by strong 

 muscles. Amongst the simpler urodeles the tongue is also 

 well developed and similar in shape and wide attachment. 

 In most of them it is nearly smooth (Proteidse, Amphiumidse). 

 But in the Amblystomidae the tongue may be broader than 

 long, and anteriorly is covered at times with coarse papillae. 

 In the higher urodeles the tongue varies from oval to much 

 elongate, and may be highly protrusible as in Spelerpes. 



Reasons for regarding certain preoesophageal glands in 

 nemerteans as homologous Tvdth the thymus and thyroids 

 have already been given (p. 4^25). Wiedersheim synopsizes 

 well as follows their origin and relation amongst craniates: 

 "The thyroid arises primarily as a median ventral divertic- 

 ulum of the pharynx in the region of the first four or five 

 visceral clefts, and in the course of development may become 

 subdivided into two lobes. In addition to this unpaired diver- 

 ticulum, paired portions, situated more posteriorly, are devel- 

 oped in mammals. 



"In the Ammocoete the smgle diverticulum, which is fined 

 by a ciliated epithelium, opens into the pharynx between the 

 third and fourth clefts, but in the adult Petromyzon the organ, 

 as in all vertebrates, loses its connection with the pharynx, 

 undergoes a modification, and gives rise to numerous closed 

 glandular vesicles enclosing the albuminous substance. 



"In the Urodela and Anura the thyroid gives rise to numer- 

 ous vesicles situated close to the anterior end of the pericardium. 



