THE ALIMENTARY CAXAL AXD ITS APPENDAGES. 129 



threads. In larvse about one-fifth of an inch long, Schindler* 

 found only eight long tubules, the usual number in Thysanura, 

 Anoplura, and Termes; but the grouping into six masses, so 

 plainly seen in the adult, throws some doubt upon this observa- 

 tion. In the adult Cockroach the long threads wind about the 

 abdominal cavity and its contained viscera. 



In the wall of a Malpighian tubule there may be dis- 

 tinguished (1) a connective tissue layer, with fine fibres and 

 nuclei ; within this, (2) a basement-membrane, between which 



c 



Fig. 72. Malpighian Tubules of Cockroach. A , transverse section of young tubule ; 

 j), its connective-tissue or "peritoneal" layer; J5, older tubule, crowded with 

 urates ; 1r, tracheal tube ; C, tubule cut open longitudinally, showing three 

 states of the lining epithelium. X 200. 



and the connective tissue layer runs a delicate, unbranched 

 tracheal tube ; (3) an epithelium of relatively large, nucleated 

 cells, in a single layer, nearly filling the tube, and leaving only 

 a narrow, irregular central canal. Transverse sections show 

 from four to ten of these cells at once. The tubules appear 

 transparent or yellow- white, according as they are empty or 

 full ; sometimes they are beaded or varicose ; in other cases, one 

 half is coloured and the other clear. The opaque contents 

 consist partly of crystals, which usually occur singly in the 

 epithelial cells, or heaped up in the central canal. Occasion- 

 all} 7 , they form spherical concretions with a radiate arrangement. 

 They contain uric acid, and probably consist of urate of soda.f 



* Zeitsch. f. wiss. Zool., Bd. XXX. 



t The contents of the Malpighian tubules may be examined by crushing the part 

 in a drop of dilute acetic acid, or in dilute sulphuric acid (10 per cent.). In the first 

 case a cover-slip is placed on the fluid, and the crystals, which consist of oblique 

 rhonibohedrons, or derived forms, are usually at once apparent. If sulphuric acid 

 is used, the fluid must be allowed to evaporate. In this case they are much more 

 elongated, and usually clustered. The murexide reaction does not give satisfactory 

 indications with the tubules of the Cockroach. 



K 



