48 OCEAN LIFE. 



it is destined to occupy. Being constantly buoyed up on all 

 sides by a dense element, it is easily supported at any required 

 altitude without much muscular effort ; but feeble limbs are re- 

 quired to guide its path through the water, and slight impulses 

 suffice to impel it forward. 



" The surface of the body of fishes, is in most instances 

 covered by numerous scales, which vary considerably in size and 

 substance in different species. The arrangement of these scales 

 exhibits considerable uniformity. Each scale is attached to the 

 fish by its anterior edge ; and the manner in which the scales 

 overlap each other in different genera, is variable, and gives an 

 appearance of form to each scale which in reality it does not 

 possess. By maceration in water, scales exhibit a series of 

 laminae, the smallest in size having been first produced : they re- 

 semble a cone, the apex of which is outward, the smallest being 

 in the centre ; hence the appearance of numerous concentric 

 lines all of the same shape, which mark the growth. 



" The fins are important, not only as organs of motion, but as 

 affording by their structure, position, and number, materials for 

 distinguishing orders, families, and genera. The membranes of 

 the fins are thin, and more or less transparent, supported by 

 slender elongated processes of bone, some of which consist of a 

 single piece, which is pointed at the end ; such fin rays are called 

 spinous rays. Others are formed of numerous portions of bone 

 united by articulations, and frequently divided at the end into 

 several filaments ; these, from their pliant nature, are called soft 

 or flexible rays, and two leading divisions in systematic arrange- 

 ment are founded on this difference in structure. The number of 

 fin rays in each fin of different examples of the same species, is 

 not always exactly alike. The names given to the different fins 

 are derived from the part of the body to which they are attached. 



" The use of the operculum or gill covers, is to close the aper- 

 ture behind the gills. The blood in fishes, while passing through 

 the gills or branchiae, receives the influence of oxygen from the 

 water which enters by the mouth and goes out by this aperture. 

 In the fishes included in the first three orders, the gills are so 

 formed, and so freely suspended, that the water bathes in its 

 passage, every part of their surface. 



