CYCLOSTOMATA 



3i 



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 



Fig. 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, suecessional tooth 

 beginning to cornify. 



' tongue ' are provided with large horny teeth of cornitied 

 epidermis. The conical horny teeth when worn out are replaced 

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

 17)). The laterahline 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- 

 lages do not meet over the neural canal in the 

 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. 17. 



Petromyzon mn ri uus. View 

 of the oral sucker, horny teeth. 

 and mouth. (After Heckel and 

 Kner, from Gegenhaur, Vergl. 

 Anat.) 



