236 VERTEBRATE ANIMALS. 



however, the nasal sacs are closed, and they do not communicate 

 by any aperture with the throat, as they do in all the higher Vev- 

 tebi'ates. The only exceptions to this rule are the Hag-fishes and 

 their allies (Jft/xinnids), and the Mud-fishes (Lepidosiren and Cerato- 

 dim). The Hag-fishes and Lampreys are further peculiar in pos- 

 sessing, alone of all Vertebrate animals, a single nasal sac, opening 

 by a single external nostril. 



As ivgards their reproductive system, most fishes are truly ovi- 

 parous, and the ovaries are familiarly known as the " roe." Some 

 fishes are ovo- viviparous, retaining their eggs within the body till 

 the young are hatched. The male organs of reproduction are com- 

 monly spoken of as the " milt " or " soft roe." 



