396 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



which never develops a hinged jaw apparatus and consequently remains 

 permanently gaping. It is this peculiarity that is expressed in the name 

 Cyclostomata. 



The lining of the buccal cavity,, and also the piston-like tongue 

 situated within it, bears remarkable spine-like teeth. These are in their 

 structure quite unlike the teeth of any other vertebrate each con- 

 sisting simply of a hollow cone of hard cornified epidermal cells 



(Fig. i74,*). 



The pharynx at the commencement of which is a velum (Fig. 175, w) 

 carries out as in other vertebrates the function of breathing, but it 



A. 1'etromyzon the Lamprey (after Starr Jordan) ; B, Myxmeihe Hagfish 

 The Cambridge Natural History), a, Anus; g, mucus-gland. 



shows striking peculiarities, some characteristic of the group as a whole, 

 others characteristic of one or other of its subdivisions. Of the former 

 the chief is that the gill-clefts instead of being literally clefts are 

 rounded sacs, opening internally from the pharynx and externally to 

 the outer surface of the body, the area of respiratory surface being 

 increased by the lining of the sac projecting into its cavity in the form 

 of prominent ridges. 



In I'xli'llos/oina, or in the larva of Petromyzon, 1 these branchial sacs 

 le.id straight from pharynx to exterior, but in Myxine and in the adult 



1 Known as tho Animncoetes larva having been given the generic name 

 'icoctes before it was recognized as the larval stage of rctromyzon. 



