PISCES. 457 



their evolution ; it has compressed their head, trunk, and tail into a spindle- 

 like form ; it has given them an easy and rapid motion, enabling them to 

 cleave the water like a rounded wedge. It has made their mode of movement 

 one of undulation, causing the sides of the fish to contract rhythmically, 

 thrusting the animal forward." Very beautiful indeed are the lines of a 

 typical swift-swimming fish, such as the common mackerel ; and in a front 

 view the sectional outline will be found to form a perfect ellipse, from which 

 project the fins as thin vertical or oblique ridges. 



Interesting and important as is the structure of fishes in general, our 

 remarks onthis subject must be the briefest. Here it is important to 

 mention that the ordinary bony fish of the present day, such as a mackerel 

 or a roach, are very specialised forms, and afford but little idea of the general 

 or primitive structural type of the class, a shark or a lung-fish being far pre- 

 ferable in this respect. Among the special organs of many, although by no 

 means all fishes, is the air-bladder, which has the power of altering the 

 specific gravity of the body to accord with that of the surrounding water. 

 Frequently this bladder is connected by means of a duct with the ossophagus, 

 and in the lung-fishes it assumes the characters of a lung. Of the external 

 coverings of fishes, the most characteristic are the scales, although these are 

 wanting in some groups and in certain members of others. The ordinary 

 over-lapping scales are classified as cycloid or ctenoid, according as to whether 

 the free margin is entire or pectinate. When they are coated with a highly 

 polished enamel-like structure, and at the same time are more or less 

 icctangular in form, they are termed ganoid. The lateral line found on the 

 sides of many fishes is formed of scales, modified partly for the purpose of 

 supplying mucus. The terminal portions of the fins namely, the fin-rays 

 are likewise structures formed in the skin ; and these rays are articulated 

 below with the interspinal bones in the bony fishes, and with the radial 

 cartilages in the sharks and rays. The paired fins comprise the pectoral and 

 pelvic ; and the median or unpaired, the dorsal, caudal, and anal. When 

 the pelvic pair are situated in advance of the pectorals, they are said to be 

 jugular in position ; when in the same line, thoracic ; and when far back, 

 abdominal. Of the internal structure of the paired fins, such observations as 

 are necessary may be deferred till the different groups are treated of ; but it 

 may be mentioned that when the rays of the caudal fin surround the hinder 

 extremity of the back-bone symmetrically, the tail-fin is said to be diphycercal. 

 When, on the other hand, the upper lobe is greatly developed at the expense 

 of the lower, while the rays are unsymmetrically, as in the sharks, it is termed 

 heterocercal. More specialised than either of the foregoing is the so-called 

 homocercal or completely forked tail, in which, although the back-bone 

 terminates in an abbreviated unsymmetrical manner, the arrangement of the 

 rays is symmetrical. Whereas in some fishes the notochord persists in its 

 original form, in others it is partially surrounded by rudimental vertebral 

 arches, and in the higher forms is completely replaced by bony vertebrae. 

 Much the same may be said with regard to the primitive cartilaginous skull, 

 which is retained in its original form in the sharks, but is completely bony in 

 the higher fishes, in which, in addition to bones developed from cartilage, a 

 number of investing so-called dermal bones likewise arise. Dermal bones, 

 too, are developed in the pectoral girdle of the latter group. Very important 

 in the skull of the bony fishes is the gill-cover or operculum, which when 

 fully developed comprises the bones respectively known as the pre-, sub-, 

 and interopercular, and the opercular proper. The gill-membrane aids the 



