URINARY ORGANS. 253 



in the genera Malapterurus, Synondontis, and others, the first verte- 

 bra is invariably provided with a large process that arises from it, 

 narrow and slender, but finally expands into a large round plate, 

 which, when at rest, is deeply imbedded upon the anterior surface of 

 the swimming-bladder. A thick muscle arises from the internal sur- 

 face of the spine of the cranium, and is inserted into this plate. When 

 it contracts, it lifts the process from off the swimming-bladder, and 

 by thus removing its pressure upon the latter, renders the air within 

 it more rarefied. If this muscle be pulled in the dead fish, and then 

 the traction withdrawn, the bony process springs back by its own 

 elasticity, and condenses again the air in the bladder. 



The swimming-bladder is almost always tightly distended with air; 

 this air consists usually of nitrogen and a very small quantity of car- 

 bonic acid gas; in some instances, however, it has been found to 

 contain nearly pure oxygen. Now, since the above-named gases 

 are diffused in the blood of the vertebrata, it is probable that they 

 have been disengaged in a free state by the vessels of the swimming- 

 bladder. Whatever be its other uses, this organ serves, for obvious 

 reasons, to facilitate the ascent and descent of Fishes in the water. 



URINARY ORGANS. 



THE Kidneys are of very large size in proportion to the body in 

 Fishes, and distinctly developed without exception in all the genera ; 

 they are placed close to each other upon the sides of the vertebral 

 column, to which they are firmly attached, but they very seldom pro- 

 ject freely into the ventral cavity from behind the peritoneum, or 

 the swimming-bladder when that organ is present. In the Osseous 

 Fishes they extend more or less through the whole extent of the 

 ventral cavity, and as far forward as the commencement of the 

 skull ; in most of these Fishes, also, they coalesce in front and be- 

 hind into a single mass. From its anterior half each kidney sends 

 off a large transverse lobe, so that the two together acquire the form 

 of a cross. In many instances the kidneys may be said to form only 

 a single mass, which is divided by the passage of the vena cava, that 

 vessels receiving much of its blood, and sometimes, as in the Cyclo- 

 stomi, appearing completely imbedded in its substance. The ureters 

 (frequently numerous) pass along the external or internal margin 

 of the renal organs, and emerge from their substance inferiorly. 

 They unite either into a common duct, or enter separately into a true 



