STRUCTURE OF THE GLIDING- WOfcMS. 8l 



two primitive kidney canals in the vertebrate embryo 

 also appeared at a very early period, shortly after the first 

 differentiation of the middle germ-layer (mesoderma). The 

 appearance of these at so early a period shows that the 

 kidneys are very important primordial organs. It also 

 shows their universal existence in all Flat- worms ; for even 

 the Tape-worms, which, in consequence of the adoption of a 

 parasitic mode of life, have lost the intestine, yet have the 

 two secreting primitive kidneys, or " excretory ducts." The 

 latter seem, therefore, to be older and of greater physiologi- 

 cal importance than the blood-vessel system, which is wholly 

 wanting in the Flat- worms. The sexual organs appear 

 in many of the Gliding- worms in a very complex form ; 

 while in others their form is very simple. Most of them 

 are hermaphrodites ; that is, each individual worm has 

 both male and female sexual organs. In the simplest 

 forms we find a te'stis in the anterior part (Fig. 185, A), 

 a single or double ovary behind (a). One of these simplest 

 existing Acrelomi, such as we find among the lowest Rhab- 

 docrela, may give us an approximate idea of the structure 

 of the Primitive Worm, which forms the sixth stage in 

 the human pedigree. 



These ancestors of the human race, which, on account 

 of their general organization, must be placed among the 

 Bloodless Worms (Acodomi}, must have been represented 

 during the Archilithic Epoch by a large number of various 

 Worm forms. The lowest of these must have been directly 

 connected with the Gastraeads (the fifth ancestral stage); the 

 most highly developed must, on the other hand, have been 

 directly connected with the Crelomati (the seventh stage). 

 As, however, our present knowledge of the Comparative 



