362 ME. H. M. BEENARD O^' THE 



For a comparative account of tlie epithelium lining these digestive diverticula and of the " peritoneal " 

 cells covering them externally, which latter are generally ahseut in Galeodes, occurring only here and 

 there, hut are common to all other Arachnids, cf. " Notes on some of the Digestive Processes in the 

 Arachnids," Journ. Royal Microsc. Society, 1893. 



The Malpig-hian tubules open into the mid-gut in the 4th segment, where the gut 

 narrows (see PL XXXIII. fig. 1, and the section on excretion, p. 380). 



T/ie Hind-gut and Stercorcd Pocket. — The passage from the mid- to the hind-gut 

 takes place in the region of the 7th segment ; it can easily be made out in sagittal sections 

 of distended specimens. The long club-shaped cells suddenly cease, and the chitin-lined 

 hind-gut commences (PL XXXIII. figs. 8, 11). The posterior portion of this hind-gut is 

 specialized into Avhat is called a stercoral pocket, which is simply a great enlargement of 

 its dorsal wall. In the contracted condition of the abdomen it reaches far forward, but 

 much less so when the abdomen is distended. 



In order to increase the surface of the stercoral pocket in Galeodes, its wall is thrown 

 into elaborate folds, between which the faecal masses are pressed, having, as I have shown 

 elsewhere (ii), digestible material absorbed out of them. Great masses of faeces are 

 retained in this pocket for such final absor2:»tion. I have only seen traces of an extremely 

 thin tesselated epithelium on the outer side (i. e. on the side tow^ards the body-cavity) of 

 the thin chitinous membrane, with minute, rather closely arranged nuclei. In addition 

 to this epithelium, there is a thin layer of muscle-fibres, but their exact distribution 

 round tlie jiocket is very difficult to make out ; I could find no such regular basket 

 arrangement as that figured by Dufour (31). 



Purther, when we remember that many of these animals live on scorching sands, where 

 an economy of fluid matter is a necessary condition of existence, Ave may well believe 

 that one function of this hind-gut is to dehydrate the faeces *. 



The anus is a large median slit, sometimes placed ventrally {Mhaa;), but, as a rule, 

 posteriorly, in the anal segment. 



The great variation in the hind-gut of Arachnids is a point of no small interest. 



The Spiders have the posterior abdominal segments much shortened; hence \ie find the stercoral 

 pocket developed dorsally, as in Galeodes, as a receptacle for the fseces f. 



In Scorjj'/o the hind- gut, commencing in the region of the 7th segment, runs through the long 

 specialized tail-segments as a straight tube, showing no enlargement as a stercoral pocket. 



In Chernes we seem to have a long coiled hind-gut, showing further a slight enlargement anterior to 

 the anal aperture. 



In Tliehjphonus, the hind-gut runs through the three tail-segments as a thin tube, while an anterior 

 enlargement functions as stercoral pocket. The greater part of this enlargement appears to me to 

 be chitin-lined, the chitin commencing a little behind the entrance of the Malpighian tubules 



* It has long been recognized that this is a conspicuous function of the mammalian hind-gut {cf. Edkins, Journ. 

 of Physiol, vol. i, p. 459). 



t Kischinouye (40) claims that the stercoral pocket of the Spiders is of mesodermal origin, and Laurie (47) 

 that that of Phrynvs is cndodermal. In view of the presence of a chitin-lined hind-gut in Oalcodes, Scorpio, and 

 Thehjiihonus reaching, at least in the two former, to the 7th abdominal segment, I think there must have been some 

 error in interpreting the phenomena {cf. note, p. 380). 



