PRIMITIVE NUMBER OF URINARY TUBES 



353 



and (3) pale transparent concretions of leucine. Crystals of taurin are also said 

 to occur. (Glaus' Zoology, p. 531.) 



Although uric acid is characteristic of the urinary tubes, yet sometimes it is 

 wanting in them, while uric acid substances in quantity occur in the fat-body or 

 in the mid-intestine. 



In the living insect the urinary tubes remove urates from the blood; "the 

 salts are condensed and crystallized in the epithelial cells, by whose dehiscence 

 they pass into the central canals of the tubules and thence into the intestine." 

 (Miall and Denny.) 



The process of excretion is carried on not only by the urinary tubes, but also, 

 as Cuenot has recently shown (1896) in Orthoptera, by the pericardial cells and 

 certain cells of the fat-bodies. In the last-named cells urates are stored through- 

 out life; the pericardial cells apparently secrete but do not store waste products, 

 which are finally eliminated by the urinary tubes, the latter constantly eliminat- 

 ing waste. 



Primitive number of tubes. Wheeler considers the primitive number of 

 urinary tubules to be six, other authors regarding two pairs as the primary or 

 typical number ; and while Wheeler agrees that the more ancestral tracheate 

 arthropods had but a single pair, Cholodkowsky supposes the primitive number 

 in insects themselves to be a single pair. This view is strengthened by the fact 

 that Scolopendrella has but a single pair (Fig. 15). 



While Peripatus has no urinary tubes, in Myriopods a single pair arises, as in 

 insects, from the hind-intestine. 



When in insects the number of these tubes is few, they are, with rare excep- 

 tions, arranged iu pairs, so that Gegenbaur and others have considered this 

 paired arrangement as the primitive one. When 

 the tubules are very numerous in the adult, as in 

 Orthoptera, the embryos and larvae have a much 

 smaller number, Wheeler stating that "in no 

 insect embryo have more than three pairs of these 

 vessels been found." We have observed 10 

 primary tubes in the embryo of Melanopus (Fig. 

 347), from each of which afterwards arise 15 

 secondary tubules. In the Termites, only, do the 

 young forms have more urinary tubes than the 

 adults. 



In Campodea there are about 16 urinary tubes 

 and in Machilis either 12 (Grassi) or 20 (Oude- 

 mans) ; but in other Thysanura the number is 

 much less, Lepisma having either four, six, or 

 eight, according to different authors, and both 

 Nicoletia and Lepismina having six, opening sepa- 

 rately into the hind-intestine. On the other hand, 

 these organs have not yet been detected in 

 Japyx. Whether they exist at all in the Collembola, which are degenerate 

 forms, is doubtful. The weight of opinion denies their existence, though they 

 may yet be found existing in a vestigial condition. They are said by Tullberg 

 and by Sommer to exist in Podura, but are of peculiar shape. 



Coming now to the winged insects, in what on the whole is perhaps the low- 

 est or most generalized order, the Dermaptera, the number is over 30, and their 

 insertions regularly encircle the intestine. (Schindler.) In the most ancient 

 and generalized family of Orthoptera, the Blattidse, Schindler detected from 

 60 to 70 tubes, but in a nymph of Periplaneta not quite 10 mm. in length he 



FIG. 347. Section of proeto- 

 of embryo locust, showing 

 origin of urinary tubes (ur.t); 

 tp, epithelial or glandular layer; 

 m, cells of outer or muscular layer ; 

 a, section of a tube. 



2x 



