10 HIGH* SCHOOL ZOOLOGY. 



Animal. In making our .sclecLioii, however, it will be desir- 

 able to clioose a form which shall be so far typical, that a 

 knowledge of the structure of its various organs will enable us 

 to interpret the nature and significance of the comparable or 

 homologous i^arts in other Vertebrates. No one animal is 

 best in every respect for tliis purpose, because there is no animal 

 which unites in itself all the characters which we recfard as 

 primitive or general. An example will render the meaning 

 of these terms plain. Most Vertebrates have five fingers on 

 the hand, and we regard that as a primitive or general ari-ange- 

 ment in comparison with that in a cow where there are two, or 

 in a horse where there is only one. Such a reduction in 

 number we regard as a specialization associated with the 

 function which the hand pei-foi-ms, and it is very much easier 

 to interpret correctly the specialized condition if we have in the 

 first place familiarized ourselves with the more primitive one. 

 Our object must then be to find some fairly primitive form, 

 which is common, easily obtained, and easily studied : our de- 

 mands in all these respects are pretty well met by the common 

 catfish, the angling for which is attended by no great dilficulties, 

 which is tenacious of life and easily kept in captivity, and 

 which finally occupies such a place in the class of the Fishes 

 that we can, after acquainting ourselves with its structui'e, 

 survey the other members of the class, and proceed to the study 

 of the higher Vertebrates. 



3. General Form. — All Vertebrates, like most Inverte- 

 brates, are bilaterally symmetrical animals, i. e., the body 

 is divisible into right and left symmetrical halves by a plane 

 passing from head to tail through the middle line of the back 

 (or dorsal surface) as well as through the middle of the lower 

 (or ventral) surface. This is the median sagittal plane ; 

 planes at right angles to it, which are parallel to the dorsal 

 and ventral sui faces, are called horizontal, while those which 

 transect the body at right angles to both are frontaL 



