AFFINITIES OF LAMPREYS 63 . 
rays, which (sometimes) bifurcate. The branchial region of 
the adult lamprey’s gullet is restricted to a pouch-like 
diverticulum (v. p. 263 and Fig. 326). A ‘sympathetic’ 
nervous system, and a ‘lateral line’ has appeared: the 
latter passes down the side in two branches, one above 
and one below the median lateral plane: its end organs 
are the pouches of nervous epithelium which in Myxi- 
noids are scattered generally over the body surface. The 
skeletal structures of the lamprey (Fig. 69, A) indicate 
well-marked advances: a stouter supporting tissue of car- 
tilage-like character has appeared ; the brain case is partly 
roofed over; neural processes, VP, a branchial basket, 
BB, and a series of mouth cartilages are especially note- 
worthy. 
Affinities of the Cyclostomes 
The relations of the group, Cyclostomi, to the earlier 
chordates, and, on the other hand, to fishes, have been by 
no means definitely established. Dohrn and others have 
suggested that the Cyclostomes are greatly degenerate, and 
are even closely akin to the recent bony fishes, as perch 
or cod. Their views have been based upon several struct- 
ural characters, notably vestigial organs, such as the ap- 
pendages at the sides of the cloacal opening of Petromyzon 
which were believed to represent pelvic fins ; and there was 
further taken into consideration the belief that the entire 
group was one of degenerate life habits. The views of these 
writers, however, do not appear to be confirmed by later 
studies, and the belief is becoming more and more general 
that Cyclostomes represent a very ancient chordate stem 
whose ancestral form is most nearly exemplified by Bdel- 
lostoma. Parasitism has been acquired to a limited degree, 
but does not appear to have affected the general characters 
