THE KIDNEYS. 359 



sight, seems of subordinate significance, but which proves, 

 by its early appearance in the animal series and in the 

 embryo, that it must be very ancient and, consequently, of 

 great physiological and morphological value. This is the 

 urinary apparatus, or kidney system, the organ-system 

 which secretes and removes the useless fluids from the body. 

 We have already seen how soon the primitive kidneys 

 appear in the embryo of all Vertebrates, long before any 

 trace of the heart is discoverable. Correspondingly, we also 

 find a pair of simple primitive kidney ducts (the so-called 

 excretory ducts or lymphatic vessels) almost universally 

 diffused in the Worm tribe, which is so rich in forms. Even 

 the lowest classes of Worms, which have as yet neither 

 body-cavity nor vascular system, are furnished with these 

 primitive kidneys (Fig. 280, nc, p. 827). It was only in 

 the fourth place, after the kidney system, that the vascular 

 system developed in our invertebrate ancestors; this is 

 plainly shown in the Comparative Anatomy of Worms. 

 The lower Worms {Acoelomi) possess no part of the vas- 

 cular system, no body-cavity, no blood, no heart, and no 

 vessels ; this is the case, for example, in the comprehensive 

 group of the Flat Worms (Plathelminthes), the Gliding Worms 

 (Tiirbellaria), the Sucking Worms (Trematoda), and the 

 Tape Worms. In the higher Worms, which are therefore 

 called Coelomati, a body-cavity (coeloma), filled with blood, 

 first begins to form , and, side by side with this, special 

 blood-vessels then also develop. These features have been 

 transmitted from the Coelomati to the four hi^rher animal 

 tribes. 



These organ-systems are common to Vertebrates and to 

 the three higher animal tribes, the Articulated Animals 



