28 CLASSIFICATION AND DISTRIBUTION OF FISHES. 



including the Sharks and Rays; the gills are fixed, and the water 

 which has been made use of in respiration passes out through a 

 series of openings corresponding in number to the gills and placed 

 on the sides at the back of the head. The skeleton in these is 

 always cartilaginous. 



II. GANOIDEI, or Enamelled Fishes, in which the gills are 

 free like those of the ordinary Fishes, and concealed by an oper- 

 culum. In the other two orders the arterial bulb is furnished 

 with thickened, but not muscular, walls ; its interior contains no 

 valves, although a pair of these organs exists at its junction with 

 the heart ; and the intestine is destitute of a spiral valve. 



III. TELEOSTEI, or Bony Fishes, with a bony skeleton, com- 

 plete jaws, and free branchiae. 



IV. CYCLOSTOMI, or Lampreys, with a cartilaginous skeleton, 

 no true jaws, a round sucking mouth, and sac-like branchial or- 

 gans, opening by a series of small holes on each side behind the 

 head. The fifth order, that of the 



V. LEPTOCARDII, or Lancet Fishes, which includes only a 

 single small species, forms the lowest step in the series of Ver- 

 tebrated animals. The vertebral column is reduced to a mere 

 gelatinous band, which supports a simple spinal cord, scarcely 

 exhibiting any dilatation at its anterior extremity ; and the heart 

 is entirely wanting, the circulation of the blood being effected by 

 the pulsation of the larger vessels. 



574. The class of Fishes is probably the most numerous of 

 the whole Vertebrated series, both in regard to the number of 

 families, genera, and species which it contains, and in regard to 

 the number of individuals of the same species. The structure 

 of a very large proportion of it has been but very imperfectly 

 investigated ; and of the habits of these animals still less is 

 known. Instead, therefore, of giving a technical description of 

 every family, such as will be found in Systematic Treatises on 

 Zoology, we shall confine ourselves to a notice of those groups 

 which are of most general interest, either on account of pecu- 

 liarities in their structure, or their importance to Man. It has 

 been calculated that, exclusive of lakes and rivers, the seas oc- 

 cupy full seven-tenths of the earth's surface. These seas yield 



