AFFINITIES OF LAMPREYS 3 



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, NP, 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 



