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 Hning of the buccal cavity, and also the piston-like tongue 

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

 structure quite unUke the teeth of any other vertebrate — each con- 

 sisting simply of a hollow cone of hard cornified epidermal cells 

 (Fig. 174, s). 



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

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



Fio. 173- 

 Cyclostomes, A. Petromyzon — the Lamprey (after Starr Jordan) ; B, Myxine — the Hagfish 

 {from 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 Bdellostoma, or in the larva of Petromyzon,^ these branchial sacs 

 lead straight from pharynx to exterior, but in Myxine and in the adult 



1 Known as the Ammocoetes larva — having been given the generic name 

 Ammocoetes before it was recognized as the larval stage of Petromyzon. 



