THE MARSIPOBEANCHII. 125 



having a single, median, external aperture. In all other 

 Vertebrata there are two nasal sacs. In the Lampreys, the 

 nasal sac terminates blindly below and behind, but in the 

 Hags {Myxine), it opens into the pharynx. In no other 

 fishes, except Leindosiren, does the olfactory apparatus com- 

 municate with the cavity of the mouth. 



The reproductive organs of the Marsipobranchii are solid 

 plates suspended beneath the spinal column, and they have 

 no ducts, but shed their contents into the abdomen, whence 



Fis. 32. 



3 



Fig. 32.— Vertical and longitudinal section of the anterior part of the 

 body of a Lamprey {Petromyzon mnrinns):—A, the cranium with 

 its contained brain ; a, section of the edge of the cartilage marked 

 a, in Fig. 30 ; Olf, entrance into the olfactory chamber, which is 

 prolonged into the CEecal pouch, o ; Ph, the pharynx ; Pr, the 

 branchial channel, with the inner apertures of the branchial sacs ; 

 31, the cavity of the mouth, with its horny teeth; 2, the cartilage 

 which supports the tongue ; 3, the oral ring. 



they pass out by an abdominal pore. In the early stages of 

 their development the Lampreys present some singular re- 

 semblances to the Amphibia. They also undergo a metamor- 

 phosis, the young Petromyzon being so unlike the parent, 

 that it was, until lately, regarded as a distinct genus— 

 Ammoccetes. But the yoimg Lampreys never possess ex- 

 ternal branchial filaments or spiracula. 



The Marsipobranchii are inhabitants of both fresh and 

 salt water. The Myxinoids are remarkable for their parasitic 

 habits — the Hag boring its way into the bodies of other 



