338 



COMPARATIVE ANATOMY 



CHAP. 



the Cray-fish they are developed in summer and are largest just before the skin is 

 cast. During this process they reach the cavity of the masticatory stomach, are 

 there ground up, dissolved, and re -absorbed. It is highly probable that they 

 yield the calcareous material for the hardening of the new after the casting of the 

 old skeleton. 



The whole intima of the fore-gut, to- 

 gether with the complicated chitinous 

 framework of the masticatory stomach, is 

 removed when the integument is shed ; it 

 is vomited from the mouth. 



B. The Mid-gut. 



This represents the endodermal portion 

 of the enteric canal, and its epithelium shows 

 clearly its endodermal origin even in the 

 adult, being differently constituted from the 

 g , ectodermal epithelium of the fore- and hind- 

 guts. The mid -gut again differs from 

 the two other divisions of the intestine 

 in having no chitinous intima answering 

 to the outer chitinous cuticle of the 

 body. It is nearly always distinctly 

 marked off from the fore- and hind-guts by 

 constrictions or valve -like arrangements. 

 Besides this its anterior end, and occasion- 

 ally its posterior end also, is marked by 

 the entrance of glandular diverticula which 

 represent invaginations of its wall. These, 

 among which the hepatopancreas of the 

 higher Malacostraca shows the greatest 

 development, we shall describe in the sec- 

 tion on the divertieula of the mid-gut. 

 The form and extension of the mid-gut 

 are very varied, while in the Entomostraca 

 and many Malacostraca it represents by far 

 the largest portion of the enteric tube, 

 running from the head to near the posterior 

 end of the body ; it has in other Mala- 



FIG. 234. Astacus fluviatilis, median section of the body, seen from the right side. The 

 thoracic feet and antennae of the left side are incompletely drawn (after Huxley), oa, Ophthalmic 

 artery (aorta cephalica) ; aa, antennal artery ; cth, cephalo-thorax ; o, lateral ostium of the heart ; h, 

 heart ; sa, sternal artery ; t, testes ; vd, vas deferens ; em, extensor muscles of the abdomen ; oaa, 

 upper abdominal artery (arteria abdominalis) ; db, abdomen ; hd, hind-gut ; an, anus ; t, telson ; 

 g, brain. (supra-cesophageal ganglion) ; 6/4, 4th thoracic foot (chelate foot) ; oe, oesophagus ; cs, cardiac 

 portion of the stomach ; ps, pyloric portion of the stomach ; bd, right aperture of the hepato- 

 pancreas into the stomach ; bm, ventral chord ; md, mid-gut ; Ir, liver (hepatopancreas) ; vdo, male 

 genital aperture ; pli, pl^, pl$, 1st, 5th, and 6th pleopoda ; uaa, lower abdominal artery ; fin, flexor 

 muscles of the abdomen. 



