168 STUDY OF NATURAL HISTORY. 



before proceeding further, in what respect these 

 may be said to differ. 



(110.) By form, or external structure, we com- 

 prehend not only the different external parts of the 

 body, and of the members thereunto attached ; but 

 all such organs as have one of their surfaces, at 

 some time or other, protruded and exposed to the eye, 

 and which may be observed without the necessity 

 of dissection. Hence it follows, that the jaws, the 

 teeth, and the mouth of quadrupeds; the bill and 

 tongue of birds ; the instmcmenta cibaria, or parts 

 of the mouth, in insects ; the external coverings of 

 the bodies of tortoises (Chelonia) and shell-fish 

 ( Testacea) ; the retractile tentaculce, where they exist, 

 of caterpillars and snails ; and the proboscis of inferior 

 animals; — all these are parts of their external ana- 

 tomy, or, as we shall hereafter say, of their form. All 

 other, — that is to say, such as are enveloped and con- 

 cealed beneath the cuticle, or that substance which 

 acts as the external protection of the animal, — relate 

 to its internal construction or anatomy. We contend 

 not for the critical accuracy of these definitions, but 

 for their general truth and convenience. It may be 

 urged, indeed, with some show of reason, that 

 nearly all teeth are internal, and that the organs of 

 the, mouth in insects cannot be studied without 

 dissection ; but it must likewise be remembered, that 

 neither one nor the other are enveloped in other 

 substances, and that their outward surface is exposed 

 to the action of atmospheric air. 



(111.) To estimate aright the respective value of 

 these two modes by which animals may be dis- 

 tinguished, becomes the first duty of the student; 



