ALIMENTARY CANAL OF ARTHROPODA. 



269 



They become organs wliicli will be considered wlien we come 

 to speak of tlie appendages of tlie mid- 

 gut. 



The hind-gut forms the shortest, and 

 generally a narrower portion of the enteric 

 tract. It is seldom widened mesially, and 

 in a few only is it provided with cEecal 

 appendages. 



The function of the enteric canal is not, 

 in all Crustacea, limited to digestion. In 

 some (Astacus, Limnadia, Daphnia) an 

 almost rhythmical taking-in and driving- 

 out of water may be observed in the hind- 

 gut, so that this portion appears to have a 

 respiratory function also. 



In many lower Crustacea the enteric 

 canal undergoes degeneration. It disap- 

 pears in the degenerate males of the para- 

 sitic Copepoda, and in some Cirripedia 

 and most Rhizocephala, where nutrition is 

 efiected by other means (cf. supra, p. 236). 



§ 209. 



Fig. 137. Digestive organs 

 of a Spider, oe Qilsopliagus. 

 c Snpraoesophageal ganglion 

 (Cerebrum). v Stomach. 

 v' Its lateral processes. 

 v" Appendages directed up- 

 wards, i Mid-gut. r Cloacal 

 widened end of the enteron. 

 lih Openings from the liver 

 into the enteron. e Urinary 

 canals (after D ages). 



The three divisions of the enteric tube 

 are distinctly marked off in the Protra- 

 cheata ; the mid-gut, which is distinguished 

 by its width, forms the largest part of it. 



The enteric tube of the Arachuida is divided into a larger 

 number of segments, except in the degenerate forms. The narrow 

 fore-gut (Fig. 137, oe) leads into a 

 mid-gut, which is generally elongated, 

 and has its more anterior portion (v) 

 frayed out into lateral ctecal sacs. 

 These are absent in the Phrynida 

 and Scorpionea. In the Aranea there 

 are five pairs of them {v'), which 

 extend to the base of the legs and 

 palps. In the Galeodea four pairs, 

 the last two of which are bifurcated, 

 extend into the appendages (feet, 

 claws, and palps) ; in the Pycnogo- 

 nida these cseca extend almost to the 

 end of the appendages (Fig. 188, li). 

 The presence of these parts greatly 

 increases the intei^nal cavity of the 

 stomach. 



In the Acarina these cfccal sacs are confined to the body ; there 

 are generally eight of them, but any diminution in number is 



Fig. 133. Digestive organs of 



Ammothoe pycnogonoides. 



a Stomach, h Cffica (after Quatre- 



fages) . 



