ox THE KESPIRATION OP FIIHES. 357 



ders, nature having furnished them with two apertures at 

 the top of the head communicating directly with th6 throat. 

 These are real nostrils, placed behind the eyes, but formed 

 to admit the water, into the mouth, whenever the throat 

 dilates. The return of the liquid in the same direction is 

 prevented by valves ; and the animal, performing the action 

 of deglutition without opening the mouth or CESophagus, 

 forces the water into the cavities of the gills. 



The mode of respiration too must be different, brut for 2d, in fishes 

 another reason, in the lampreys and gastrobronchi, the only others „r to 

 species destitute of truejaws ; and which have been classed solid bodies, by 

 perhaps erroneously among the vertebrate animals, fron^ eirmou 

 which they differ considerably, as I intend to show in a 

 paper that I shall soon have the honour of laying before 

 the class. 



Many of these species are parasitical : they fasten them- 

 selves by the mouth to other fishes, or fix themselves on 

 stones and other solid bodies by the help of this part and a 

 kind of suction. The water therefore must necessarily enter 

 by some other orifice, that of the mouth being closed. In 

 fact we perceive on their head, or in their lips, a patulous 

 orifice opening to a canal, which directs the water into tho 

 throat as often as a vacuum is produced in it. The motion 

 of deglutition that follows forces this water to penetrate 

 into peculiar cavities, in which the bronchias float; without 

 this fluid being able to issue out by the same orifice, though 

 Bloch aspribes this faculty to them, and, as well as Ronde- 

 letius, has figured this sort of fountain in several of the 

 plates of his superb work. 



All these fishes then are circumstanced precisely the same These similar 

 as the batrachian reptiles, since they inspire water at the [JJJ^^ j^- ^ 

 nostrils, and by deglutition force the fluid into the pulmo. 

 nary cavity, from which it is afterward discharged by a 

 different orifice. 



We find too some peculiarities of conformation in other Peculiarities 

 species, the respiration of which is as it were arbitrary, at JIonTs^n^ somt 

 least as to the duration of the three periods, or movements, d<^£^ree arbi- 

 tjiat compose its mechanism, namely inspiration, degluti- *'*'^^' 

 t)on, and expiration. Besides, these circumstances pro- 



duc^, 



