80 



tul)ulos, wliifli are somewhat spaisely distriliiited in the 

 anterior jiart of the oio-an. hnt are veiv aljimdant in the 

 thickened posterior portion. They are greatly convoluted 

 tubules of varying' diameter opening into the ureters. It 

 is somewhat remarl;a1)k^ that Malpighian Ixxlies ai'C very 

 (lifHcuIt to tiiid, and indeed seem to be absent, in some 

 j)arts at least, of the kidney. This condition is connected 

 with the vascular arrangements of the organ. By far the 

 gieater portion of the blood entering it is venous, and the 

 arterial supply is very scant}'. Two, or at most thiree, very 

 small vessels originating directly in the dorsal aorta enter 

 at the dorsal surface, and the common genital artery 

 (.4. f/en., fig. 22) gives off several very fine arterial twigs 

 which ramify in the posteiior portion. The whole arterial 

 blood supply is very small compared with the amount of 

 venous bh)od entering by the renal portal veins. 



'idle lymphoid tissiu^ which is so frequently met with 

 in the kidneys of hshes is most abundant in the middle 

 and anterior regions of the plaice kidney. It consists of 

 very small cells, suppcnted by reticular ccmnective tissue, 

 and filling up the interspaces between the blood vessels 

 and the uriniferous tubules. Groups of pigment granules 

 are scattered throughout this lymphoid tissue and give the 

 organ its black appearance. They are small lounded 

 granules of variable diameter, and of a greenish-bhud; 

 coh)ur. They lie freely among the lymphoid cells. 



The Pronephros and Head Kidney (Text-iig. '1). — 

 The kidney in P/euronerfes is a mesonephros, and its 

 jiaiied ducts are segmental or archinephric ducts. The 

 most common mode of origin of these structures in 

 Teh'osts is l)y a longitudinal evagination of somatopleure 

 foiining a groove which afterwards closes by constriction 

 of its li])s, giving rise to a tube. Mcintosh and Prince 

 state, however [loc. tit.), that in (iadoids and Pleuro- 



