iv. 8 EXCRETION IN LAMPREYS 93 



power of the blood. Haemopoietic tissue occurs in the intestinal wall 

 of the larva and this has been regarded by some as representing the 

 spleen. In the adult the blood-forming tissue lies below the spinal 

 cord and in the kidney. White corpuscles resembling lymphocytes 

 and polymorphonuclear cells occur, produced by lymphoid tissue in 

 the kidneys and elsewhere. However, there is no distinct system of 

 lymphatic channels. 



8. Urinogenital system of lampreys 



The excretory and genital systems of vertebrates consist of a series 

 of tubes opening from the coelom to the exterior and serving to carry 

 away both excretory and genital products. This plan of organization is 



Fie. 58. Diagram to show arrangement of the pronephros in a freshly hatched 



lamprey. 

 g. gonad; pr. pronephros; prd. pronephric duct. (After Wheeler.) 



quite different from that found in amphioxus and represents a new 

 acquisition by the vertebrates. It is not clear whether the excretory 

 or genital component of the complex is the primary one, nor indeed 

 why they are associated. The gonads develop from the walls of the 

 coelom in all animals possessing that cavity; some hold that the coelom 

 represents an enlargement of a sac that at first served purely as a 

 gonad. Genital ducts leading from the coelom to the exterior are 

 common in invertebrates, and we may guess that at their first appear- 

 ance the urinogenital tubules of vertebrates served only for genital 

 products. 



The conversion of these tubules to excretory purposes may have 

 been a result of the adoption of the freshwater habit. The blood of 

 lampreys, when in fresh water, contains a higher concentration of salts 

 than the surrounding water. Little is known about the condition in 

 sea lampreys, where blood is probably hypotonic to the sea. When in 

 the river the animals must deal with the tendency for water to flow in. 

 This water must be removed without losing salt ; accordingly in most 

 freshwater animals, including vertebrates, we find some system by 

 which the separation can be achieved. 



