576 



PHYLUM CHORDATA I CLASS CYCLOSTOMATA 



of this shows a strong muscular cylinder surrounding a 

 cartilage. 



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 dis- 

 tinctly developed, there is only a hint of the complicated 

 basket-work which supports the gill-pouches of the lam- 

 prey ; but the tongue, the barbules, etc., are supported by 

 cartilaginous rods. The tail is protocercal. 



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



NTT 



Fig. 324. — Median longitudinal section of anterior region of 

 Myxine. — After Retzius and Parker. 



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

 N.C., nasal capsule ; BR., brain ; S.C., spinal cord ; N., notochord ; G., 

 gut ; T., cartilage of " tongue " ; Ml)., muscle ; NTT., posterior part 

 of nasal sac ; T.P., a tooth plate ; M.T., median tooth onroof of mouth 

 (M.). 



the cerebrum and cerebellum are little more than rudi- 

 mentary. It is much compressed, with practical oblitera- 

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

 that 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 



