518 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 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. 
Fic. 273.—Median longitudinal section of anterior region of 
Myxine.—After Retzius and Parker. 
&., Barbule; 4, nasal aperture ; V7., nasal tube with rings of cartilage ; 
NC., nasal capsule; BR., brain; SC., spinal cord; ., notochord ; G., 
gut; 7., cartilage of “tongue”; M7U., muscle; WTT., posterior part of 
nasalsac; 7.,atooth plate; A77., median tooth on roof of mouth (/Z.). 
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 Adedlostoma, an American relative) 
opens dorsally at the apex of the head, and. communicates 
posteriorly with the pharynx by a naso-palatine duct. It 
