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 znstrwmenta cibaria, or parts 
of the mouth, in insects; the external coverings of 
the bodies of tortoises (Chelonia) and shell-fish 
( Testacea); the retractile tentacule, 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; 

