344 THE INVERTEBRATA 



but only rarely to colour (wave-length). In light of high intensity or 

 on a light-absorbing (e.g. dull black) background it expands; in light 

 of low intensity or on a light-dispersing (e.g. dull white) background 

 it contracts. Different pigments are affected to different degrees, and 

 thus both the degree and the pattern of the coloration of a sensitive 

 species (notably, for instance, of many prawns), changes with its 

 surroundings — usually, in nature, in such a way as to render the animal 

 inconspicuous. The response to intensity of illumination is due to 

 direct action of the light upon the chromatophores and will thus take 

 place even in blinded animals; the response to background depends 

 upon the eyes. The eyes, however, do not act through nerves to the 

 chromatophores, but by causing certain endocrine glands to pass 

 hormones into the blood. 



The alimentary canal (Figs. 233, 236, 244, 278, 289) is with very 

 rare exceptions straight, save at its anterior end, where it ascends 

 from the ventral mouth. The/or^ gut and hind gut (stomodaeum and 

 proctodaeum), lined with cuticle inturned at the mouth and anus, 

 leave a varying length of mid gut (mesenteron) between them. The 

 intrinsic musculature, sometimes supplemented by extrinsic muscles 

 running to the body wall, is strongest in the fore gut, whose lining 

 sometimes develops teeth or hairs. In the Malacostraca (Fig. 228) 

 these elements become a more complex proventriculus ("stomach"), 

 with a ''gastric mill" and a filtering apparatus of bristles which 

 strains particles from the juices of the food, the mill and filter 

 being often in separate "cardiac" and "pyloric" chambers. The 

 mid gut usually bears near its anterior end one or more pairs of 

 diverticula ("hepatic coeca"), which serve for secretion and absorp- 

 tion and may branch to form a "liver". This gland, however, 

 unlike the liver of vertebrates, forms all the enzymes necessary 

 for the digestion of the food and absorbs from its lumen the products 

 of digestion. It stores the reserves in the form of glycogen and fat. 

 Occasionally there is an anterior median dorsal coecum. Coeca 

 are also sometimes found at the hinder end of the mid gut: these 

 are more often median. In a few cases the hind gut is absent and the 

 mesenteron ends blindly. In the Rhizocephala and the monstrillid 

 copepods (p. 374) the alimentary canal is absent throughout life, 

 for these animals absorb through the skin during the parasitic 

 period enough nutriment to last through an entire life history. 



Digestion is extracellular. The fore gut is frequently the seat of 

 mechanical processes, and sometimes of chemical action by juices 

 secreted by the mid gut diverticula, but never of absorption. The 

 latter process as well as most of the chemical work is performed by 

 the mid gut, including the hepatic diverticula. In the hind gut the 

 faeces are passed to the anus, being in some entomostraca sheathed in 



