THE WONDERS OF LIFE 



inflating it again. The lungs were formed by the adapta- 

 tion of the blood-vessels in the wall of the swimming- 

 bladder to the interchange of gases. In the oldest living 

 lung-fishes {ceratodtis) it is still a simple sac {mono- 

 pneumones = onQ-hingQ6.) ; in the others the simple gullet- 

 cavity divides early into a pair of sacs {dipnenmoncs, 

 two-lunged). The wind-pipe {trachea — not to be con- 

 fused with the organ of the same name in the tracheata) 

 is formed by the lengthening of their stalk and strength- 

 ening of it with cartilaginous rings. At the anterior end 

 of the trachea we find already formed in the amphibia 

 the larynx, the important organ of voice and speech. 



The function of removing unusable matter is not less 

 important to the organism than breathing. Just as 

 breathing gets rid of the poisonous carbonic acid, so the 

 kidneys remove fluid and solid excreta in the shape of 

 urine; these are partly acid (uric acid, hippuric acid, 

 etc.), partly alkaline (urea, guanine, etc.). In most of 

 the coelomaria special organs for removing these would 

 be superfluous, as this is accomplished (like breathing) 

 by the stream of water that is constantly passing 

 through the whole body. But with the platodes we be- 

 gin to find important excretory organs in the nephridia, 

 a pair of simple and ramified canals which lie on either 

 side of the gut, and open outward. These primitive 

 renal canals are transmitted by the platodes to the 

 vermalia, and by these to the higher stems of the 

 coelomaria. In the latter they generally open by 

 special funnels into the inner body-cavity, which 

 serves as first receptacle for the urine. Their outer 

 opening sometimes (primarily) goes through the outer 

 skin at the back (excretory pores), sometimes (second- 

 arily) to the rectum, and so out through the anus. 

 The oldest articulates, the annelids, have a pair of 

 nephridia in each segment of the body; each renal 

 canal, or segmental canal, consists of three sections, an 



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