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136 of its relative dimensions being easily stated. The same remark applies to its form, although it most commonly inclines to oval. Its situation is more constant, 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 cæcum. From its having been noticed in all perfect aquatic insects, M. Léon Dufour regards this appendage 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 cæcum: 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