THE KIDNEY 35 



body wall like the scuppers of a ship, or into a common 

 longitudinal groove in the skin. (See Figure 5a.) 



These primitive nephric tubules probably also served 

 to carry off the eggs and sperm, which were shed freely 

 by the gonads into the coelomic cavity; embryonically, 

 the gonads are derived from the coelomic membrane 

 adjacent to the tissue which forms the nephric tubules, 

 and this suggests that reproduction and excretion have 

 had, from far back in vertebrate history, a close aflfiha- 

 tion. 



This segmented Tddney'— if we may so designate a 

 dozen or more pairs of primitive nephric tubules— is such 

 as one might expect to find in a segmented animal in 

 which the coelomic cavity participated in excretion and 

 where a relatively feeble stream of coelomic fluid would 

 serve to wash the excretion out of the tubules; and it 

 may be accepted that it was with this meager equip- 

 ment that the protovertebrate tried to enter the brackish 

 lagoons or fresh-water rivers and lakes of the Paleozoic 

 continents. 



The armor of the ostracoderms served in its passive way 

 to reduce the influx of water into the body to a mini- 

 mum, but the gflls and the membranes lining the mouth 

 could not be insulated in this manner. Moreover, the 

 ostracoderms must have swallowed considerable quanti- 

 ties of water when eating microscopic food, as do many 

 living fresh-water fishes. Consequently the early fresh- 

 water invaders had to compensate for this irreducible 

 influx of water by increasing the rate of its excretion. 

 It appears that no better way could be devised to get 

 this water out of the body than to have the heart pump 

 it out; and the simplest way to do this was to install, 

 close to the open mouths of the pre-existing tubules, a 

 filtering device in the form of a tuft of permeable capil- 

 laries (see Figure 5b). Under the hydrostatic pressure 

 supplied to the arterial blood by the heart, water was 

 filtered through these capillaries and drained out of the 



