470 CYCLOSTOMATA. 



The Skin. 



The skin is scaleless, slimy, and pigmented. Its structure, 

 like that of Myxine, is complex. Sensory structures occur 

 on the head and along the sides. 



Muscular System. 



The muscle segments are well marked. The suctorial 

 mouth and the rasping " tongue " are very muscular. 



The Skeleton. 



The skeleton is wholly cartilaginous. The notochord 

 persists unsegmented, but its firm sheath forms rudimentary 

 neural arches. The skull is imperfectly roofed. There are 

 no distinct jaws, but a cartilaginous ring supports the lips of 

 the mouth. There is a complex basket work around the 

 gill pouches, and it is likely that its elements correspond to 

 visceral arches. Fin rays support the dorsal and caudal fins, 

 and other skeletal parts occur about the " tongue." The 

 caudal end of the notochord is quite straight. 



Nervous System. 



The brain has the usual parts, but is small and simple ; 

 the roof of the fore brain is composed of non-nervous 

 epithelium ; there is a distinct pineal body ; the oral part of 

 the hypophysis is developed from in front of the mouth, in 

 close connection with the involution of epiblast which 

 forms the nostril. The spinal cord is flattened ; the anterior 

 and posterior roots of the spinal nerves do not unite ; there 

 is no sympathetic system. 



Though the larva sometimes receives the name of " nine- 

 eyes " which expresses a popular estimate of the branchial 

 apertures it is blind, for the eyes are rudimentary and 

 hidden. In the adult they rise to the surface, and are fairly 

 well developed. The ear has only two semicircular canals 

 instead of the usual three. The single nasal sac does not 

 open posteriorly into the mouth as it does in Myxine ; 

 though prolonged backwards it ends blindly. Its external 

 opening is at first ventral, but is shunted dorsally. 



Alimentary System. 



The oral funnel, at the base of which the mouth lies, 

 has numerous horny teeth. It is applied to the lamprey's 



