FISHES. 11 



absence of the glands that secrete milk, from the sucking dolphins 

 and cetaceans. LiNN^US, in the last edition of his arrangement, 

 referred the cartilaginous and some other fishes, to which he erro- 

 neously ascribed, in addition to gills, lungs also, to the class of the 

 Am'pMhia. This position was rejected by subsequent writers, and 

 Gmelin, in the thirteenth edition of the Systema Naturce, restored 

 these animals to the fishes. 



Fishes, as is well known, occur in water alone ; and, however 

 some or even many species from all the other classes of animals 

 live in water, still fishes form the greatest number of the in- 

 habitants of water, at least among vertebrate animals ; so that it is 

 not strange that, by the uninitiated and in common language, the 

 appellation of fishes should be transferred to other water-animals. 



Let us first consider shortly the external structure of fishes; 

 this will afibrd us, at the same time, the opportunity of explaining 

 some terms which are employed in the description of these animals. 



The body of fishes may be divided into head, trunk, and tail. 

 The head is an immediate continuation of the trunk ; fishes have no 

 proper neck, since the respiratory organs are seated under the head, 

 and the thoracic cavity immediately succeeds to that of the mouth, 

 or is even confluent with it. Hence the form of the body is very 

 simple, usually attenuated gradually towards the two extremities. 

 In some the hinder end is, as it were, cut ofi", as in the sun-fish 

 {Orthagoriscus). In the rays the tail is much narrower than the 

 trunk. 



The body of most fishes is compressed laterally {corjnis com- 

 j^resswn s. cathetoplateum) , so that the section is an oval, of which 

 the back forms the broadest end. In others, as in the rays, it is 

 depressed or flat {corjnis depressum s. plagioplateum) ; in others, 

 still, it is cylindrical, as in the eels ; in others almost spherical, as 

 in some species of the genus Diodon ; in some, finally, angular, sur- 

 rounded by flat or sliglitly concave surfaces, meeting in three or 

 four projecting edges, as in the so-named Cofi'er-fishes, the genus 

 Ostracion. 



In general, the body is covered with scales {corjms squamosum). 

 Sometimes the scales are small, and the smooth slimy skin seems 

 to be naked, as in the eels ; in some fishes, however, as in the 

 cyclostomes, the scales are really wanting {corpus nudum s. alepi- 

 dotum) . 



