136 ANATOMY AND PHYSIOLOGY 
of its relative dimensions being easily stated. The 
game remark applies to its form, although it most 
commonly inclines to oval. Its situation is more con- 
stant, for it always occupies the hinder segment of 
the abdomen. It frequently has a pouch-shaped 
appendage, originating at its commencement, which 
sometimes appears quite distinct from it, although 
their cavities are always connected with each other. 
In this case there is no valve to separate it from the 
ctecum. From its having been noticed in all perfect 
aquatic insects, M. Leon Dufour regards this appen- 
dage as a kind of swimming bladder ; but to this view 
there are great objections, among others, that which 
arises from the consideration that air could not have 
ready access to it. It is more probable that it is 
in some way subservient to defecation. The rectum 
is nothing more than the terminal portion of the 
caecum: it is always short, and provided with a 
Sphincter or strong annular muscle, which closes 
the aperture when not in action. Its muscular tunic 
is very thick, its parietes very variable in form, and 
it is from them that the excrements acquire their 
particular figure. 
Having thus noticed the divisions of the great 
nutrimental tube, leaving some peculiarities to be 
noticed when treating of the respective tribes in 
which they appear, it is now' necessary to advert to 
its appendages, which were stated to consist of the 
salivary , biliary , and urinary vessels. Insects are 
seldom provided with glands, properly so called, as 
a necessary consequence of the w r ant of a perfect 
