290 VERTEBRATE!) ANIMALS. 



rate the oxygen from the atmospheric air contained in the water; and hence, if water is 

 deprived of this air, or impregnated with deleterious gases, Fishes cannot live in it. As 

 little can they bear the return of water entering at the gill-openings, and escaping by 

 the mouth ; for if a fish is held so that the water is made to pass in this direction, it is 

 as speedily drowned as if it were an air-breathing animal]. The blood is brought to 

 the gills by the heart, which thus answers to the right ventricle of warm-blooded 

 animals ; and from the gills it is sent to an arterial trunk, lying immediately upon the 

 under side of the back bone, which trunk is the left or systematic ventricle of the heart, 

 and sends the blood throughout the body of the fish. 



Living habitually in water, which is of very nearly the same specific gravity as their 

 bodies, Fishes have no weight to bear, but merely to propel themselves through the 

 water ; and their form and their organs of motion are all adapted to this one purpose, 

 though varying in the species. In many, there is under the spine a membranous air- 

 bladder, which the fish can expand or contract at pleasure ; and this is understood to 

 alter its gravity, and enable it to suspend itself at any depth in the water. [Many 

 fishes, wanting this apparatus, have, however, nearly the same habits as others which 

 are possessed of it.] 



Progressive motion is effected by the tail striking alternately right and left against tbe 

 water, [for which purpose the flexure of the spine is lateral, whereas in the other Verte- 

 brata generally, the principal flexure is vertical], and perhaps the jet of water thrown 

 backward from the gill-openings may assist. Thus a fish has but little use for extremi- 

 ties ; and the parts analogous to legs and arms are accordingly very short, terminating in 

 a number of rays analogous to fingers and toes, and these, covered by membranes, form 

 what are termed fins. The fins answering to arms are called pectorals, and those 

 answering to legs ventrals ; and besides these there are often fins on the back called 

 dorsal, behind the vent called anal, and on the extremity of the tail called caudal. 



The texture of the fins is important in classification. If the rays consist of single 

 bones, whether stiff or flexible, they are said to be spinous ; and if they consist of a 

 number of jointed pieces, divided at their extremities, they are called soft, or articulated. 



The pectorals are attached to two bones immediately behind the gills, and answering 

 to the scapulars, which bones are sometimes imbedded in the muscles, or attached to the 

 spine, but generally to the bones of the head. The pelvis rarely adheres to the spine ; 

 and it is often in advance of the belly, and attached to the bones of the shoulders. 



The vertebra; have their proximate surfaces concave, and filled with cartilage, which 

 forms the joints, and is generally continued by an aperture through the centre of each 

 vertebra. Spinous processes, upwards and downwards, support the muscles, and main- 

 tain the vertical position of the body ; but, as far as the cavity extends, the downward 

 processes are wanting, and there are transverse processes, to which the ribs are some- 

 times soldered by cartilages. 



The head varies much in form, but in general consists of the same number of bones 

 as in the other Vertebrata, — a frontal of six pieces, parietals of three, occipitals of five, 

 and five of sphenoid and two of each temporal bone, are included in the composition of 

 the cranium. 



Besides the brain, which is disposed as in Reptiles, Fishes have nodes or ganglions 

 at the base of their olfactory nerves. The nostrds are simple cavities at the end of the 

 muzzle, always pierced with two holes, and lined by a regularly-plaited pituitary mem- 



