DIGESTION 43 



an elaborate musculature. The lumen bears rows of curved spines by 

 means of which the pollen grains are filtered out from the nectar and 

 passed on to the stomach while the clarified nectar is retained in the 

 crop. In the flea it is lined with long needle-like teeth directed back- 

 wards; during digestion it contracts rhythmically, driving the teeth 

 backwards into the mid-gut; at the same time the mid-gut contents 

 are driven up against it by anti-peristaltic waves, and in this way the 

 blood corpuscles are pounded and disintegrated. 



In most insects the fore-gut is invaginated to some extent into the 

 mid-gut, and this invagination, surrounded by the anterior or cardiac 

 region of the mid-gut, is also commonly referred to as the 'proventri- 

 culus'. The invaginated fore-gut is called the 'oesophageal valve', 

 though it seems never to function as a valve (if, by that word, is in- 

 tended a mechanism which automatically allows movement in one 

 direction only), for the occlusion of the passage between fore-gut and 

 mid-gut is secured always by a sphincter muscle, and the invagination 

 of the fore-gut has quite another significance. 



The peritrophic membrane 



The gut of insects contains no mucous glands ; the boluses and hard 

 particles of food are therefore not lubricated as they are in the in- 

 testine of vertebrates; and the epithelial cells obtain protection in 

 another way. As was first observed by Lyonet (1762), they are 

 separated from the gut contents by a delicate membrane, composed 

 of chitin mixed or combined with protein, known as the peritrophic 

 membrane. It is this membrane which takes the place of mucus. 

 Chito-protein is a mucopolysaccharide; mucus is a variable mixture 

 of mucoprotein and mucopolysaccharides - so that chemically there 

 is no great divergence here. 



The peritrophic membrane is freely permeable to digestive en- 

 zymes and to the products of digestion; experimentally it has been 

 shown to be permeable to all dyes save those with the largest col- 

 loidal particles. In its primitive state it is a somewhat indefinite affair 

 formed by the condensation over the surface of the food of certain 

 constituents in the mid-gut secretion; and it arises in this way to a 

 greater or less extent in many existing insects, where it is made up of 

 a number of concentric layers, all of which are chitinous. These 



