GENERAL CHARACTERISTICS. 319 



rous vertebrates, if we consider every piece a distinct bone. 

 The jaws are armed with teeth, and in many cases these 

 are placed in all parts of the mouth, and even in the gul- 

 let. The spinal column is made up of vertebrae which 

 are concave at each end, and the cavities which thus 

 occur between the vertebrae are filled by a soft membra- 

 nous and gelatinous substance, which extends from one 

 cavity to another, through a hole with which each verte- 

 bra is pierced. The spinal column bends with -perfect 

 freedom laterally, but not in a vertical direction ; and it 

 is chiefly by the lateral motions of the tail and body that 

 Fishes are propelled ; although some swim principally by 

 the undulation of the dorsal fin. The fins are employed 

 mainly in balancing and directing. The muscular sys- 

 tem is highly developed. Their flesh, except certain 

 muscles which are deep red, is paler than that of Birds or 

 Mammals, and in some cases is pure white. One large 

 and complicated muscle on each side, and filling up the 

 space from the head to the tail, furnishes the principal 

 motive power. The brain is exceedingly small, and sel- 

 dom fills the cavity in which it is situated. The senses 

 of smell, sight, and hearing are conferred on Fishes by 

 organs analogous to those of other vertebrates, and are 

 arranged in nearly a similar manner. The nostrils are 

 simple cavities at the end of the muzzle, almost always 

 perforated by two holes. The position, direction, and 

 size of the eye in Fishes are almost endlessly varied. 

 With few exceptions, the eye has no motion ; the iris 

 neither contracts nor dilates, and the pupil is never al- 

 tered, whatever be the quantity of light. The ear of 

 Fishes is enclosed on every side in the bones of the head, 

 and consists merely of a sac, representing the vestibule, 

 and of three membranous semicircular canals. In the 

 former are suspended small bodies, generally of stony 

 hardness. All the vibrations which reach the ear of 



