COMPARATIVE MORPHOLOGY OF THE GALEODID.E. 363 



(PI. XXXIV. fig. 2). As in the hind-gut of Galeodts, tlie chitinous wall is thrown into countless folds, 

 which are found caked with coagulum, and thus much obscured. Laurie attributes tlie whole of this 

 stercoral pocket to the mid-gut. 



In Phryniis, my sections (not very well preserved) seem to show a similarly sudden change in the 

 7th segment, from mid-gut to hind-gut, as is seen in Galeodes. 



The relative lengths (measured iu segments) of the different portions of the alimentary canal in the 

 ancestral Arachnid may be stated as follows : — 



The oesophagus ran through two segments. 



The mid-gut had originally four pairs of segmental diverticula in the cephalothorax (reduced to 

 one pair in Scorpio and C/iernes), and presumably seven pairs in the abdomen. It thu.s ran through 

 eleven segments. We shall see further (footnote, p. 307) that there is reason to believe that there 

 were originally 12 pairs of diverticula. 



The hind-gut, therefore, ran from the 14th or 15th segment to the end of the body^ and has been 

 variously specialized in adaptation to the progressive shortening of the abdomen. 



Eevii'w of the Alimentary System. — In endeavouring to understand these variations in the morphology 

 of the alimentary canal in the Arachnids, we have to ajjpeal to its physiology. 



The typical food of the Arachnids is essentially fluid, i. e. the body-juices sucked from prey seized, 

 held, and crushed in front of the beak or mouth. These juices are, in nearly all cases, carefully 

 strained by special apparatus in front of or in the mouth. By means of a special puniping-apparatus 

 (developed primarily iu the beak) this liquid food is forced into the mid-gut and its diverticula, 

 giving rise even to accessory diverticula where space among the tissiies could be found for them. 

 This method of filling the mid-gut with fluid requires regulation. We find, for instance, that but a 

 small quantity of this food runs into the ceplialotboracic diverticula, whose serious distension would 

 interfere with the highly-developed musculature of this region of the body. Undue distension of the 

 cephalothoracic diverticula is perhaps prevented by their muscular tissue (PI. XXXII. fig. 19), which 

 appeal's to be much more powerful than that of the abdominal diverticula. After simply filling the 

 cephalothoracic diverticula, the food flows on into the abdomen, where the highly specialized diverticula 

 are filled to their fullest capacity. Galeodidae are often found with enormous abdomens, reminding one 

 of females distended by eggs ; on dissection, the distension is found to be due to food. The abdomen 

 of Spiders can even be seen to swell while they drink ; and carmine particles mixed in the water are 

 found at the tips of the diverticula (Bcrtkau). Most remarkable of all, the ticks, when once attached 

 to a host, having practically an unlimited supply of food, distend so enormously that their skins are 

 specially strengthened, as much, perhaps^ to prevent them from bursting themselves as from being easily 

 burst by the scratching action of the host. 



With regard to the mechanical movements of the distensible bag-like receptacle for the liquid food, 

 when there are rigid tergitcs aiul steniites, the segments telescope into each other, while latcrallv 

 there is a strong flexible membrane {Galeodes, Scorpio, Chernes, Pedipalpi). In the Spiders, many 

 Acari and Phalangids, the whole abdominal integument seems to be equally extensible. In Scorpio 

 only seven, and in Thelyphonus nine, segments are capable of distension [cf. PI. XXIX. fig. 14). 



The constriction of the mid-gut by means of the diaphragm takes place in Galeodes [cf. PI. XXXIII. 

 figs. 2, 3) J Scorpio, and Spiders (PI. XXXIII. fig. 6), and very probably in all Arachnids which have 

 diaphragms or waists. This is an arrangement quite in keeping with the distension of the abdomen by 

 forcing liquid food into it by means of the pumping-apparatus. The constriction serves to prevent the 

 food from flowing back into the cephalothorax. In Galeodes and the other Arachnids with the sucking- 

 apparatus in front of the nerve-mass, the long, narrow oesophagus, as it passes through the brain, is 

 supplied with circular muscles which would prevent the fluid from returning into the pumping-apparatus. 

 In the Spider Lycosa, and, according to Schimkevitch, in Epeira, there is a W'Cll-developed sphincter 



