Ill 



THE LAMPREYS AND THEIR ALLIES 



THE relations of the more primitive chordates to the 

 true fishes have not been considered in the present dis- 

 cussion. A brief account, however, must be given of the 

 Cyclostomes, or Marsipobranchii, which are represented in 

 the recent lampreys and hags. 



The three prominent forms of Cyclostomes are figured 

 on a following page (Figs. 70-72, A-D). They are eel- 

 like in shape, but are lacking both in paired fins and in 

 an under jaw. Their mouth is of a rounded form, and 

 is suctorial ; when closing, its lateral margins draw to- 

 gether. Their skeleton is of the simplest character, mem- 

 branous rather than cartilaginous ; its elements are never 

 more highly differentiated than those shown in the ac- 

 companying figure (Fig. 69, A). 



Bdellostoma is shown in surface view in Figs. 70 and 

 72 A, and in sagittal section in Fig. 69. It is looked 

 upon as the most archaic form of the living Cyclostomes. 

 Barbel-like structures surround its mouth region ; its nasal 

 canal (Fig. 69, N and C) has a forward opening at the 

 snout, and a hinder one piercing the roof of the pharynx, 

 -a very exceptional character in fishes; its tongue, stud- 

 ded with rows of rasp-like teeth,* may be greatly everted, 



* The teeth of Myxinoids are cuticular structures, and may well have been 

 evolved within the limits of the group. Beard has homologized them with the 

 teeth of sharks, but his determination of the presence of true enamel has not 

 been confirmed (Ayers). 



57 



