16 INVERTEBRATE ZOOLOGY. 



Sub-cladus I. Cyclostomata. 



Mouth circular, suctorial, without trite jaws. Within the mouth are 

 "teeth" of pure epidermic formation, and not homologous with the teeth of 

 higher vertebrates. Paired limbs fail. The gill arches connect with each 

 other and form a sort of basket-work in the pharyngeal region. 



Sub-cladus II. Gnathostomata. 



Mouth slit-like, transverse, furnished with jaws, which are originally modi- 

 fications of the first visceral arch. The integument develops horny plates, 

 formed from both epidermis and cutis, some of which become localized on 

 the jaws as true teeth, others by fusion giving rise to the so-called " dermal " 

 bones, which reinforce the skeleton. Paired limbs typically present. Appen- 

 dicular skeleton develops, consisting of girdles about the body with which the 

 skeleton of the free-limbs articulates. 



