ALIMENTARY CANAL OF ARTHROPOD A. 271 



portion, and at the same time undergoes the largest amount of 

 differentiation. 



When most simple the enteric tube traverses the coeloin in a 

 straight line ; in this point the Myriapoda resemble the lower stages 

 of the Insecta. In the Myriapoda the hind-gut is seldom very long ; 

 when it is, it is looped. A portion of the mid-gut still more rarely 

 takes part in fomiing this loop (Grlomeris) . 



The simple character found in most larva3 docs nut persist in 

 most divisions of the Insecta ; in most the separate portions get to 

 vary greatly in character, and these changes are, as a rule, correlated 

 with the appearance of the imago-stage. They are really due to the 

 great differences in the relations of the animal to external conditions, 

 which now commences. 



The mode of life seems to be of the greatest influence on the 

 general configuration of the alimentary canal, and there is often a 

 greater length of tube in the phytophagous insects — as, indeed, 

 frequently happens elsewhere in the Animal Kingdom — than in 

 those which live on animal matters. The character of the food is 

 also another cause which comes into consideration ; we find, that 

 is, simpler characters in the intestines of those insects which live on 

 fluids, while there is greater complication of structure in those 

 which subsist on solid food. 



These characters are seen most strikingly on comparing the 

 digestive tube of an insect larva with that of a developed insect; 

 we see, for example, a caterpillar (Fig. 139) provided with a wide 

 tube running straight through the body. This arrangement is 

 adapted to the enormous amount of food which is taken every 

 day ; while the butterfly, which takes in only a little food, and 

 that fluid, has a canal which is longer indeed, but much more 

 delicate (Fig. 141). 



Further, the difference between the enteric canal of the perfect 

 insect and that of its larva depends on a change in the relations of the 

 various divisions of it ; while, in the larval stage, the mid-gut is 

 ordinarily the largest portion, it becomes gradually shorter, and the 

 hind-gut is proportionately increased in length. Thus the enteric 

 tube ceases to be a straight one. The increase in length of the 

 separate portions produces curvatures of the tube, which is now 

 longer than the whole body-cavity, and these may lead to the 

 formation of various coils in it. These affect the mid- and hind- 

 guts, while the fore-gut most completely retains its primitive course 

 (cf. Figs. 139-141). 



Fresh differentiations in the various portions are connected with 

 these, so that the boundaries between them often disappear. The 

 mid- is distinguished from the fore-gut by its glandular character, 

 and where the latter has appendages, or diverticula, they servo for 

 the reception and the further comminution of the food ; in the latter 

 case they have the characters of a gizzard. Finally, the hind-gut is 

 characterised by the Malpighian vessels which open into it. 



Plateau, P., Kecli. sur la phenom. de la digest, et sur la structure de l'appareil 

 digest, c'nez les Myriapodes. Mem. Acad. Belg. XLII. 



