CRUSTACEA 307 



setae, standing on the front of the head and known zs frontal organs, 

 are found in many crustaceans and are supposed to be sensory. They 

 are present as two papillae in the Nauplius larva (Fig. 247, ten.). The 

 nuchal sense organ or ** neck organ " of many branchiopods is a group of 

 cells on the upper side of the head containing refractive bodies and 

 connected to the brain by a special nerve. Its function is unknown. 



As is well known, most crustaceans are pigmented. The pigments 

 are of various colours — red, orange, yellow, violet, green, blue, 

 brown, black, etc., though not all are found in any one species. The 

 majority of them are lipochromes, though the brown and black are 

 melanins. For the most part they are contained in branched cells 

 (chromatophores), but some of the blue, and perhaps of certain others, 

 is diffused in the tissues. The chromatophores may lie in the epi- 

 dermal layer, in the dermis, or in the connective tissue of deeper 

 organs. Their behaviour has been studied in various malacostracans. 

 The pigment is often caused to expand or contract, which it does by 

 flowing into and out of their processes. In this it is affected by light, 

 responding both to intensity of illumination and to the nature of the 

 background, but only rarely to colour (wave-length). In light of high 

 intensity or on a light-absorbing (e.g. dull black) background it ex- 

 pands; 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 chromato- 

 phores 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 the secretion 

 of certain endocrine glands to be poured into the blood. 



The alimentary canal (Figs. 222, 225, 233, 244, 264, 275) is with 

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

 from the ventral mouth. The fore 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. 217) 

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

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

 strains coarse particles from the food which has been sufficiently 

 finely divided, the mill and filter being often in separate " cardiac " and 

 " pyloric " chambers. The mid gut usually bears near its anterior end 



