404 THE EVOLUTION OF MAN. 



been incidentally mentioned. (Cf. Chapter XVII.) It is 

 found almost universally distributed, not only in the higher 

 animal tribes, but even in the more primitive Worm tribe. 

 Among the latter it even occurs in the lowest and most 

 imperfect known Worms the Flat Worms (Plathelminthes) 

 (Fig. 18-4, nc, p. 80). Although these acoelomatous Worms 

 have no body-cavity, no blood, no vascular system, they 

 always have a kidney system. It consists of a pair of 

 simple or of branched canals, lined by a layer of cells, which 

 absorb useless juices from the tissues and discharge them 

 through an external skin-opening (Fig. 184, mrri). Not 

 only the free-living Gliding Worms (Turbellaria), but also 

 the parasitic Sucking Worms (Trematoda), and even the 

 still more degraded Tape Worms, which, in consequence 

 of their parasitic habit of life, have lost their intestinal 

 canal, are all provided with these " kidney canals " or primi- 

 tive kidneys. Usually these canals in the Worms are called 

 excretory organs, and in former times they used to be called 

 water-vessels. Phylogenetically they must be regarded as 

 highly-developed pouch-like skin-glands resembling the 

 sweat-glands of Mammals, and, like these, developed from 

 the skin-sensory layer. (Cf. Fig. 210, n, p. 198, and Fig. 214, 

 p. 202.) 



While in these lowest unsegmented Worms only a single 

 pair of kidney ducts is present, in the higher segmented 

 Worms these ducts exist in greater numbers. In Ringed 

 Worms (Annelida), in which the body is composed of a 

 great number of segments, or metamera, a pair of these 

 primitive kidneys (hence known as segmental organs, or 

 canals) exists in each separate segment. In this case, also, 

 the canals are very simple tubes, which, on account of their 



