THE URINARY TUBES 



349 



vessels may have survived in myriopods and perhaps do exist in 

 insects as urinary tubes, and also occur in many of the Arachnida, 

 and thus are characteristic of each important class of land arthro- 

 pods, but are either wanting or are very rudimentary or much modi- 

 fied in the marine classes, notably the Crustacea and Merostomata 

 (Limulus), where they are represented by the 

 shell-glands of Copepoda, green glands of the 

 lobster, and the brick-red glands of Limulus. 



In the earliest tracheate arthropod, Peripatus, 

 these tubes are well developed and are highly 

 characteristic, each segment behind the head 

 bearing a pair (Fig. 4, so 4 -so 9 ). It has been sug- 

 gested by some, but not yet proved, that the 

 urinary tubes of insects are morphologically the 

 same as the segmental organs of worms and of 

 Peripatus ; but there are no facts directly sup- 

 porting this view, and, as Sograff states, it is a 

 pure hypothesis and can only be confirmed or 

 disproved by very detailed researches on the 

 development of the urinary tubes of myriopods 

 and of insects. Others regard them as probably 

 homologous with the tracheae, since they have a 

 similar origin. As, however, they arise in the 

 embryo as outgrowths of the proctodeeum they 

 may have arisen in myriopods and insects inde- 

 pendently, and not be vermian heirlooms. 



While in worms and in Peripatus a pair of 

 these segmental organs occur in each segment, in 

 insects this serial arrangement is not apparent ; 



FIG. 348. Digestive 



those with a purely excretory function are not canal of Peria maxima . 



I, upper lip; mh, buccal 



segmentally arranged, with outlets opening cavity; ap, common end 



i.i c it, i,- j of sa l lval y ducts (ay) ;. 



externally, but arise as outgrowths of the hind- o, oesophagus ; *, *, saii- 



. . . , , vary glands, arranged seg- 



mtestme or proctodseum oi the embryo, not mentally ; i, cca of chyil- 



i i -M i mi i j_i stomach ; Iff, their lig-a- 



bemg segmentally arranged. I he place oi their ments of attachment ; mp, 



-,-i , i j T v v j jr urinary tubes ; r, rectum ; 



origin is usually the dividing line between the a f, ahai orifice! After 

 mid and hind intestine (Fig. 343, mp) ; this Imhof ' from 8harp ' 

 applies to Scolopendrella (Fig. 15, urt) as well as to insects. 



The urinary tubes are usually long, slender, blind, tubular glands 

 varying in number from two to over a hundred, which generally 

 arise at the constriction between the mid and hind intestine, and 

 which lie loosely in the cavity of the body, often extending towards 

 the head, and then ending near the rectum (Figs. 301, 310, vm). 



