ICHTHYOPSIDA. 235 
fishes possess the essential parts of the organ of hearing, they 
possess no external ears, and in no case is there any direct com- 
munication between the ear and the outer world. In the second 
place, the organs of smell consist of a double cavity lined by a 
mucous membrane folded into numerous plaits, into which water 
is admitted, usually by two distinct apertures or nostrils. Be- 
hind, however, the nasal sacs are closed, and they do not com- 
municate by any aperture with the throat, as they do in all the 
higher Vertebrates. The only exceptions to this rule are the 
Hag-fishes and their allies ((/7yxinozds), and the Mud-fishes 
(Lepidosiren). 
As regards their reproductive system, keer fishes are truly 
oviparous, 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 reproduc- 
tion are commonly spoken of as the “milt” or “soft roe.” 
