498 



CYCLOSTOMATA. 



The skeleton.- -The skeleton is wholly cartilaginous. 

 The notochord persists unsegmented within a firm sheath, 

 the skull is a simple unroofed trough, jaws are not 

 distinctly developed, there is only a hint of the complicated 

 basket-work which supports the gill-pouches of the lamprey ; 

 but the tongue, the barbules, etc., are supported by cartila- 

 ginous rods. The end of the notochord in the tail is quite 

 straight (protocercal or diphycercal). 



Nervous system.- -The brain has the usual parts, but 

 the cerebrum and cerebellum are little more than rudiment- 

 ary. It is much compressed, with practical obliteration 

 of the ventricles. The fore-brain seems to agree with that 



M 



FIG. 238. Median longitudinal section of anterior end 

 of Myxine. After Retzius. 



N., nostril ; <5., barbule ; B., brain ; nc/i., notochord ; G. , gullet ; T., tongue 



muscle ; M. , mouth cavity. 



of " Ganoids " and Teleosteans in having a non-nervous roof. 

 The spinal cord is somewhat flattened, and is sheltered 

 simply by fibrous tissue. Throughout at least a portion of 

 the cord there are two dorsal roots for each ventral root. 

 The union of dorsal and ventral roots is only partial, and 

 there is no sympathetic system. There is no lateral line 

 system. 



The eye is without lens, cornea, iris or muscles, and is 

 hidden beneath the skin ; the optic nerves do not cross 

 until they enter the brain ; the ear has only one semi- 

 circular canal. The single nasal sac (with paired folds of 

 olfactory epithelium in Bdellostoma, an American relative) 

 opens dorsally at the apex of the head, and communicates 

 posteriorly with the pharynx by a naso-palatine duct. It 



