364 MR. H. M. BERNAED ON THE 



for constricting the passage immediately behind the sucking-apparatus. In the Spiders the constriction 

 by the muscles in the waist must therefore be primarily to relieve the cephalothoracic diverticula from 

 pressure when the abdomen is fully distended. 



We can thus understand the progressive rudiraentatiou of the cephalothoracic diverticula seen in 

 many Arachnids. Their presence is more or less a hindi'ance to the musculature, which is as highly 

 specialized in the thorax as the mid-gut is in the abdomen. Further, the comparatively feeble develop- 

 ment of the epithelia of the cephalothoracic diverticula is, as above suggested, referable to the limited 

 amount of food which they receive. 



One further arrangement is necessary, and this is to prevent the liquids being forced through the 

 central canal of the mid-gut into the hind-gut or stercoral pocket. As the pressure must be very great 

 to drive the food to the tips of the innumerable diverticula, distending them to their utmost, there 

 must be some arrangement to keep it from escaping into the hind-gut. 



In Galeodes, we find (PL XXXIII. fig. 1) that the central canal narrows greatly in the fourth 

 abdominal segment. Here, then, the canal might be constricted, perhaps by its own circular muscles, 

 especially if fsecal masses were present and helped to choke up the passage (in fig. 5 two masses of fseces 

 are seen in the passage). In this way, it is obvious that the food would be driven most easily into the 

 anterior pair of openings, and from these, along the lateral canals, into the diverticula. We find a 

 similar narrowing of the canal in Thelyphonus , sufficient, if faeces were present, to force the fiuid to the 

 end of the persistent lateral diverticula. Further, the singular arrangement above described, and shown 

 in PI. XXXIII. fig. 7, is clearly for constricting the extreme end of the mid-gut. In hungering and 

 contracted specimens we find the end of the mid-gut closed by folds of the hind-gut (fig. 9) . 



In Scorpio, the fresh liquid food is probably prevented from escaping posteriorly by muscular 

 constriction of the hind-gut, assisted by the presence of faecal masses in the central canal. In a small, 

 contracted specimen of Euscorpio, I found that the canal, at the junction of the mid- and hind-guts, made 

 a loop which would assist in arresting the progress of fluid. 



In the Pseudoscorpions, the fluid would be readily pumped into the large primary diverticula. The 

 long narrow hind-gut is generally filled with faeces. 



The Fceces. — The waste products of digestion, the fsecal " crystals," which, as I have 

 shown (it), are substantially identical in all Arachnids, would find their way back from 

 the anterior diverticula straight into the central canal, in which faecal masses are found 

 in considerable quantities. Posteriorly to this first and only pair of apertures, however, 

 the fgeces must travel forward along the lateral canal to the anterior opening. The 

 raising of the abdomen at right angles to the body might perhaps assist in 

 bringing the faeces forward towards these apertures ; but how the fseces themselves 

 travel up those diverticula which depend on each side of the body (PI. XXXIII. fig. 5) is 

 a problem which I have been entirely unable to solve. The difficulty becomes still 

 greater when the epithelial cells leave the walls of the diverticula, the tips of which become 

 mere bag-like receptacles for fsecal crystals. The circular muscles, which are easily 

 demonstrable, may perhaps bring about some kind of peristaltic action. If longitudinal 

 muscles are also present on the abdominal diverticula, as they are on the cephalothoracic, 

 the process would be facilitated. 



This problem presents itself in all Arachnids : How do the faeces find their way from the tips of 

 the diverticula back into the central canal? The Spiders appear partly to have given up the attempt, 

 and some pi'oportiou of the fscal " crystals " appear to pass out through the walls of the diverticula 

 (especially at their tips), and are apparently carried away by the Malpighian vessels (i i). 



