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Lab.). The essential part of the labyrinth is the Renal 

 tube (tu.), the cavity of which is connected in front with 

 the end sac and behind with the bladder. The roof of the 

 renal tube is in close contact with the floor of the end sac. 

 It may be conceived that the lumen of the renal tube was 

 primitively quite simple, so that in such a condition of 

 things the ventral part of the antennary gland would 

 show none of the complicated structure which we 

 designate the labyrinth. The complexity has been 

 produced in two ways. As already mentioned, the floor of 

 the end sac sends downwards numerous branched tubes, 

 the cavity of each tube being connected with that of the 

 end sac, and its walls being lined by the squamous 

 epithelium typical of the end sac. The floor of the end 

 sac is closely applied to the roof of the renal tube, so that 

 these prolongations push before them the epithelium of 

 the renal tube, at the same time breaking up its lumen. 

 Invaginations also appear in the ventral and lateral walls 

 of the renal tube, giving rise to partitions across the 

 lumen of the tube known as trabeculae. These ventral and 

 lateral ingrowths are not caused by the extension of the 

 end sac. In sections the dorsal ingrowths of the end sac can 

 always be distinguished from the ventral ingrowths by the 

 fact that the former appear to be lined by two rows of 

 epithelial cells — the squamous epithelium of the end 

 sac, carrying before it the epithelium of the renal tube, 

 the two only being separated by a narrow blood sinus. 

 The ventral ingrowths are only lined by the epithelium of 

 the renal tube, and enclose portions of connective tissue 

 which have been drawn in from tissue surrounding the 

 gland. The epithelium of the renal tube (tig. 59, e. tu.) is 

 distinctly columnar, and the protoplasm has a finely 

 striated appearance. The cells are lined by a thin border, 

 which is generally described as a cuticle. 



