CYCLOSTOMATA 



flesh of the prey, and draw it into the alimentary canal. There are 

 no paired fins or girdles. The skin is very slimy. 



No dermal skeleton whatever is present ; but the mouth and 



Fic. 16. 



Section of developing teeth of Petromyzon marinus, L. (After Warren, Q.J.M.S.) 1, 

 functional epidermal tooth ; 2, epidermis ; 3, dermal nutritive papilla ; 4, successional tooth 

 beginning to cornify. 



' tongue ' are provided with large horny teeth of cornified 

 epidermis. The conical horny teeth when worn out are replaced 

 by new cones from below (Warren [480a], Beard [34] (Figs. 16 and 

 17)). The lateral-line organs lie superfici- 

 ally exposed, on the head and trunk, not 

 sunk in a canal. A continuous, persistent, 

 and unconstricted notochord extends from 

 the infundibular region to near the end of 

 the tail. It secretes two sheaths : an outer 

 thin elastica externa, and an inner thick 

 fibrous sheath without cells (Fig. 36). 



The purely cartilaginous mesoblastic 

 skeleton is in a very rudimentary condi- 

 tion, and is more developed in the lampreys 

 than in the hag-fishes. The cartilage is of 

 peculiar and variable structure, with rela- 

 tively little matrix, Schaffer [382]. The 

 axial skeleton in the Petromyzontia (not in 

 Myxinoids) consists of a dorsal series of paired 

 neural arches, and of somewhat irregularly 

 developed interneural arches. These carti- and moutnr"(Aft'er"Heckei 

 lages do not meet over the neural canal in the *"".') fro 

 trunk-region, and alternate with the nerve - 



roots. Schauinsland [384] considers that the anterior cartilage 

 corresponds to the intercalary (interneural or interdorsal) of higher 



FIG. IT. 



Petromi/zon marinus. View 

 of the oral sucker, horny teeth. 



