460 COMPARATIVE ANATOMY CHAP. 



III. The Musculature. 



The arrangement of the muscles in the body and their relation to 

 the exoskeleton is the same in the Antennata as in the Crustacea (cf. 

 Crustacea, p. 331). The musculature seems to be broken up into a 

 very great number of single muscles, which are arranged in a definite 

 manner suitable for moving the segments, the regions of the body, the 

 limbs and their separate joints, the mouth parts, the ovipositors, stings, 

 etc. The greater part of the muscles of the body can be traced back 

 to a paired system of dorsal and ventral intersegmental longitudinal 

 muscles. While in the Myriapoda, in accordance with the homonomous 

 segmentation of the body, the musculature is repeated in all the trunk 

 segments, in the Hexapoda the musculature is very differently developed 

 in the head, thorax, and abdomen. The musculature of the thorax is 

 very strong, as might be expected from the fact that its 3 segments 

 carry the limbs and wings. The wing 1 muscles generally take a dorso- 

 ventral course in the lateral portions of the thorax. The most import- 

 ant parts among them are played by the elevators and depressors. 



The musculature is transversely striated. 



IV. The Enteric Canal. 



The mouth lies in the head between the mouth parts ; the anus 

 always in the terminal segment of the abdomen. The enteric canal, 

 in most Myriapoda and in the Apterygota, runs straight through the 

 body and thus is not longer than the body. In the Pterygota, on the 

 contrary, it generally forms more or less marked loops which are 

 wanting or not so strongly developed in the larva. It everywhere 

 falls into the 3 already known divisions : the fore-gut, which comes 

 from the ectodermal stomodseum; the endodermal mid -p gut and 

 the hind -gut, coming from the ectodermal proctodseum. These 3 

 divisions are generally distinct. Each of them can be further sub- 

 divided, especially in the Hexapoda, where special organs in the form 

 of diverticula are always to be found. Tubular and pouch -like 

 diverticula of the hind-gut (wanting only in a few Apterygota) are 

 especially characteristic of the Antennata. They appear in varying 

 numbers, function as excretory organs, and have received the name of 

 the Malpighian vessels. The salivary glands and the spinning glands 

 of the larvae, both of which emerge at or near the mouth, have already 

 been described. 



Myriapoda. The enteric canal runs straight through the body ; only in the 

 Glomeridce is it coiled in its posterior part. The mid-gut has numerous short hepatic 

 tubes. At the beginning of the hind-gut 1 or 2 pairs of long Malpighian vessels 

 enter ; these run along the gut, frequently winding round it. 



Hexapoda. Each of the 3 principal divisions of the enteric canal may present 

 various modifications, except in the Apterygota and the larvae of those Inlecta whose 



