270 W. SAVILLE-KEXT, CONTRIBLTTIONS TO A 



that the caecal system is represented by one antero-median and 

 two prolonged lateral elements, and it is indeed found that this 

 formula for the most part obtains where superficially the dis- 

 position is quincamcial, the apparent subdivision into two inde- 

 pendent caeca being caused by each lateral caecum, as viewed 

 from above, dipping down out of sight into the body cavity 

 and again reappearing on the surface. There are nevertheless 

 many types, such as Diplodontas Jilipes Dug., in which the 

 number of caecal prolongations is greatly augmented and ex- 

 hibit a stellate plan of disposition more nearly conforming with 

 that of the true spiders. The colour of the caecal diverticula 

 varies in different species from tawny yellow to deep chocolate- 

 brown, such tint being due to the cellular elements of a glandular 

 nature which closely invest the caecal walls throughout their 

 length. There is every reason to believ^e that these coloured 

 glandular caeca fulfil a digestive purpose, and indeed a similar 

 morphological and histological composition characterises the 

 digestive or formerly so-called hepatic organ both in many 

 -arthropoda and other higher invertebrates. In the same 

 manner that the hepatic element invests the wall of the stomach 

 or mid-gut, it is found that the excretory structure correlated, so 

 far as it is possible to determine, with a renal organ, is applied 

 to that of the hind-gut or third division of the alimentary tract. 

 With the hind-gut it describes, in the first place, a straight 

 <50urse along the anal line from the subcentral stomach to the 

 posteriorly and ventrally located anal aperture. At the com- 

 mencement of its course it likewise, almost invariably, gives 

 off two arm-like lateral diverticula, which, combined with the 

 straight median element, confer upon the hind-gut with its 

 renal investment, as seen from above, a characteristic Y- c>r 

 T-shajDed contour. The granular substance that enters chiefly 

 into the composition of the excretory organ varies from chalky 

 white through yellow and orange to deep vermilion, and being 

 in many instances of considerable breadth, contributes extensively 

 to their gay colouration. In numerous species the simple Y" or 

 T-shaped contour of the excretory organ is rendered more com- 

 plex by additional diverticula, which may be developed from the 

 extremities of the lateral arms, from the centre of the main gut, 

 or from its anal termination. Though subject to slight variation, 

 even among individuals of the same species, the broad plan and 



