136 ANATOMY AND PHYSIOLOGY 



of its relative dimensions being easily stated. The 

 same remark applies to its form, although it mos 

 commonly inclines to oval. Its situation is more con- 

 stant, for it always occupies the hinder segment 

 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 

 caecum. From its having been noticed in all perfec 

 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 want of a perfect 



