4o6 CYCLOSTOMATA. 



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 are on the surface, and fairly 

 well-developed. The ear has only two semi-circular 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 internal teeth. It is applied 

 to the lamprey's victim, and adheres like a vacuum sucker ; 

 the toothed " tongue " works like a piston ; both flesh and 

 blood are thus obtained. From the floor of the pharynx a 

 groove is constricted off", which is apparently homologous 

 with the thyroid of higher Vertebrates. 



From the gullet of the young larva seven gill-pouches open 

 directly to the exterior ; but in the adult this gullet becomes 

 wholly a respiratory tube. It forms a cul-de-sac, opening 

 anteriorly into the gullet of the adult, which is a new 

 structure. At the junction of the respiratory tube with the 

 gullet of the adult, lie two flaps or vela. 



The rest of the gut is straight and simple, with a single- 

 lobed liver, but without a pancreas. There is a slight fold 



