290 VliUTEEKATtJj ANIiiALb. 



rate the oxvfcen from tlie atmospheric air contained in tlie "water; and lience, if water is 

 deprived of tliis air, or imjireg;nated witli deleterious gases, tishes cannot live in it. As 

 little can tbey bear the return of \vatcr entering at the gill-openings, and escaping by 

 the mouth : fur if a fish is held so that the water is jnade to pass in this direction, it is 

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

 the gills by the heart, whicli thus answers to the ri^bt 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 bodjr 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 uicrely to |)ro|iel themselves through tlie 

 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 spiine 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 a2:iparatus, have, however, nearl)' the same habits as others which 

 are possessed of it.] 



Progi-essive motion is elf'ected by the tail striking alternately right and left against the 

 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 m 

 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 ventral-; ; 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 eauilal. 



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 jiieces, divided at their extremities, they are calk'd snj't, or articulated. 

 The pectorals arc attached to two bones inimcdiatelv 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 jielvts rarely adheres to the spine; 

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



The vertebrce 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. Sjjinous jirocesses, upwards and downwarils, support the muscles, and main- 

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

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

 times soldered by cartilages. 



The head varies much in form, Ijut iii general consists of the same number ol bones 

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

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

 tlie cranium. 



Besides the lirain, wdiich is disposed as in I'ejitiles, Fishes have nodes or g;inglions 

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

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



