816 



CLASS V. AEACHNIDA. 



opens into the endodermic portion (mesenteron) of the alimentary 

 canal. This gives off, whilst still within the cephalothorax, 

 two lateral diverticula each of which gives origin to five caeca 

 (Fig. 537, 8). The caeca of the anterior pair pass forward to- 

 wards the head and in a few cases they are known to fuse together. 

 The posterior four pairs of caeca pass into the four pairs of coxae 

 of the legs and sometimes turn back again and end under the 

 nerve-mass (8). They do not anastomose. The mesenteron is then 

 continued as a spacious tube through the abdomen where it 

 receives the ducts (3) of a large " liver " which occupies a good 

 deal of the space of the abdomen ; finally it opens into a rectum 

 which bears dorsally a large stercoral pocket or sac (5) in which 

 the faeces accumulate. The rectum ends in the anus ; it is 



18 



FIG. 537. Diagram of a Spider, Araneus (Epeira) diadematus, showing the arrangement 

 of the internal organs x about 8. 1 Mouth; 2 sucking stomach; 3 ducts of liver; 

 4 malpighian tubules ; 5 stercoral pocket ; 6 anus ; 7 dorsal muscle of sucking stomach ; 

 8 eaecal prolongation of stomach ; 9 cerebral ganglion giving off nerves to eyes ; 10 sub- 

 oesophageal ganglionic mass ; 11 heart with three lateral openings or ostia ; 12 lung sac ; 

 13 ovary ; 14 acinate and pyriform silk glands ; 15 tubuliform silk gland ; 16 ampullilorm 

 silk gland; 17 aggregate or dendriform silk glands; 18 spinnerets or mammillae; 19 

 distal joint of chelicera ; 20 poison gland ; 21 eye ; 22 pericardium ; 23 vessel bringing 

 blood from lung sac to pericardium ; 24 artery. (From Warburton.) 



lined with chitin and is proctodaeal in origin. Just where the 

 mesenteron unites with the proctodaeum a pair of malpighian 

 vessels (4) pour their secretion into the intestine. The various in- 

 vestigators who have described the development of spiders are 

 not in accord as to the origin of these excretory tubules, but the 

 most definite account attributes them to the mesenteron. 



Coxal glands, which are excretory in function and which are 

 believed to represent nephridia opening on the one hand into 

 an end-sac and on the other to the exterior, are \vell developed 

 in somespiders, e.g. the common house-spider, Tegenaria derfiamii, 



