COMPARATIVE MORPHOLOGY OF THE GALEODID^. 359 



typical four pairs of diverticula, we find in Scorpio only one pair, which have often been taken for 

 salivary glands. 



In the Pscudoscorpions also there is only one pair of diverticula in the cephalothorax, but m these 

 animals the diverticula are not so specialized ; they are only shallow lateral outpushings of the mid-gut, 

 which might easily run into one another, so that the segmental divisions may have become obliterated. 



Passing on from tliese four pairs of diverticula, the last two, as above stated, shomng a 

 tendency to branch, we find in Galeodes, immediately in front of the diaphragm, numerous 

 small branched accessory diverticula (PI. XXXII. fig. 18, ad), resembling those behind the 

 diaphragm. These are interesting, as they seem to show the action of the pumping- 

 apparatus in causing diverticula to appear irregularly, w^herever, in fact, there is room 

 among the tissues for their development. 



Behind the diaphragm, the whole cavity of the abdomen which is not occupied by the 

 genital glands, the stercoral pocket, tracheae, &c. is filled to distension witli diverticula. 



The Abdommal Diverticnia. — The abdominal diverticula are hooped by fine circular 

 muscles, like those of the cephalothorax. These circular fibres are 4-5 ju apart in the 

 contracted normal condition of the diverticulum, -with the epithelium in situ, but 8-15 ^ 

 apart in distended diverticula, with the epithelium disorganized by food {of. ii). 



The distinction often drawn between the mid-gut diverticula in the cephalothorax and 

 the " liver " diverticula in the al)domen is erroneous. As I have shown elsewhere (il), 

 the epithelium of these diverticula throughout the w'hole mid-gut from the posterior 

 end of the oesophagus to the commencement of the hind-gut is essentially similar, and 

 is throughout digestive in its function. The differences which appear in the epithelia of 

 the cephalothoracic portions of the mid-gut and of the abdominal are not differences in 

 kind ; they are due to the fact that the latter have far more food pumped into them to 

 digest. The diverticula throughout the Arachnida are typically mere extensions of the 

 digestive surface, and are nowhere converted into hepatic or pancreatic glands. 



This uniformity of the epithelium in the diverticula throughout the whole alimentary canal is true of 

 Galeodes, the Spiders *, Scorpio, and Cherries. Lavirie ('47) claims a difference between the epithelium 

 of the cephalothoracic and abdominal diverticula in Thelyphonus. I would suggest that the difference 

 noticed by him is accidental, and due chiefly, as above suggested, to the state of the contents and 

 consequent activity of the cells. 1 can see no essential difference between the two in my own sections of 

 Thelyphonus. 



It is a difficult and tedious oj)eration to dissect out the connections of the diverticula 

 with the central canal in the abdomen. I do not believe that any clear idea of the 

 arrangement of the diverticula can be obtained in this way. It was only after carefully 

 reconstructing serial sections of the abdomen tliat the foUoAving remarkable arrangement 

 was discovered. 



As shown in PI. XXXIII. fig. 1 {cf. also the section fig. 4), close behind the diaphragm, 

 a large group of diverticula open into the alimentary canal on each side somewhat dorsally. 

 There is no difficulty in finding the apertures of these diverticula in sections, on accoimt 

 of their size and importance. 



* In the Spiders there appear to bo a few highly-specialized cells scattered irregularly in the epithelium of the 

 .abdominal diverticula, which are not simply digesting-cells. 



