Similar cæcal tubes, sometimes very numerous and densely clustered, are attached to the stomach in many Crustacea and Arachnida. The researches of Hoppe Seyler, Krukenberg, Plateau, and others have established the digestive properties of the fluid secreted in them, which agrees with the pancreatic juice of Vertebrates. _The Malpighian Tubules._ The Malpighian tubules mark the beginning of the small intestine, to which they properly belong. They are very numerous (60–70) in the Cockroach, as in Locusts, Earwigs, and Dragon-flies; and unbranched, as in most Insects. They are about ·8 inch in length, and ·002 inch in transverse diameter, so that they are barely visible to the naked eye as single threads. In larvæ about one-fifth of an inch long, Schindler[130] 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 observation. In the adult Cockroach the long threads wind about the abdominal cavity and its contained viscera. [130] Zeitsch. f. wiss. Zool., Bd. XXX. [Illustration: Fig. 72.--Malpighian Tubules of Cockroach. _A_, transverse section of young tubule; _p_, its connective-tissue or “peritoneal” layer; _B_, older tubule, crowded with urates; _tr_, tracheal tube; _C_, tubule cut open longitudinally, showing three states of the lining epithelium. × 200.] In the wall of a Malpighian tubule there may be distinguished (1) a connective tissue layer, with fine fibres and nuclei; within this, (2) a basement-membrane, between which 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. Occasionally, they form spherical concretions with a radiate arrangement. They contain uric acid, and probably consist of urate of soda.[131] In the living Insect the tubules remove urates from the blood which bathes the viscera; the salts are condensed and crystallised in the epithelial cells, by whose dehiscence they pass into the central canals of the tubules, and thence into the intestine.