iv. 8 



EXCRETION IN LAMPREYS 



95 



mid 



duct, which reaches back to an aperture near the anus. Close to each 

 funnel there develops a tangle of blood-vessels, the glomerulus (Figs. 

 59 and 6o). Presumably the osmotic flow of water into the body is 

 relieved by the pressure of the heart-beat forcing water out from the 

 glomeruli into the coelomic fluid, whence it is removed by the funnels, 

 with the aid of their cilia. The tubules become longer and twisted 

 after hatching and may perhaps serve for salt-reabsorption. 



These anterior funnels constitute 

 the pronephros. As the animal 

 grows they are replaced by a more 

 posteror set, the mesonephros. 

 There is, however, a gap of 

 several segments in which no 

 tubules appear (Fig. 6o), a strange 

 and unexplained discontinuity, 

 common to all vertebrates. The 

 pronephric tubules gradually dis- 

 appear and finally in the adult all 

 that remains of the organ is a mass 

 of lymphoid tissue. Meanwhile the 

 mesonephros develops as a much 

 larger fold, hanging into the coelom 

 and containing very extensive wind- 

 ing tubules. These do not open to 

 the coelom (at least in the adult) but 

 each to a small sac, the Malpighian 

 corpuscle, which contains a portion 

 of the coelom and the glomerulus. 

 This is obviously a more efficient method for allowing the heart to 

 pump excess water out of the blood and down the tubules. The latter 

 themselves have become greatly elongated and make up the main 

 bulk of the organ (Fig. 6i). The segmental arrangement is there- 

 fore much obscured and as extra glomeruli are added it disappears 

 completely. The mesonephros extends at its hind end as the animal 

 grows, until it forms the adult kidney, a continuous ridge of tissue 

 reaching back to the hind end of the coelom. Besides the excretory 

 apparatus the kidney also contains much lymphoid tissue and fat, and 

 it probably plays a part in the formation and destruction of red and 

 white corpuscles. 



The gonads are unpaired ridges medial to the mesonephros. Pri- 

 mordial germ-cells, set aside very early in development, migrate into 



Fig. 6i. Transverse section of kidney of 

 hampetra. 



gl. glomerulus; mid. middle section of 



tubule; pr. proximal region of tubule; term. 



terminal region, opening into W.d. Wolffian 



duct. 



(After Krause.) 



