III 
THE LAMPREYS AND THEIR ALLIES 
Tue 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, VV 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 
