5i8 



CYCLOSTOMATA. 



The skeleton. The skeleton is wholly cartilaginous, 

 The notochord persists unsegmented within a firm sheath, 

 She 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 tail is protocercal. 



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 

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



MT 



NTT 



FIG. 273. Median longitudinal section of anterior region of 



Myxine. After Retzius and Parker. 



B. t Barbule; N., nasal aperture ; NT., nasal tube with rings of cartilage; 

 ;VC., nasal capsule; BR., brain ; SC., spinal cord ; N., notochord ; ., 

 gut ; T. t cartilage of " tongue" ; MU., muscle ; NTT., posterior part of 

 nasal sac ; TP. t a tooth plate ; MT. , median tooth on roof of mouth (M. ). 



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 



