VERTEBRATES. 51 



reality, large lymph spaces, for minute lymph vessels open freely into them, so that 

 a fluid injected into the ccelom is readily taken up by the small communicating 

 lymphatics. 



In many lower vertebrates the coelom communicates with the outside by abdominal 

 pores. These are not always homologous, although they may perform, in certain in, 

 stances, the same function — the discharge of the sexual products. Although the 

 nature of these structures is by no means clear, it appears that they are, in certain 

 cases, remnants of the ducts of the sexual organs, and thus more properly referable 

 to the urogenital system, which we shall now briefly consider. 



THE UROGENITAL SYSTEM. 



In many Invertebrata, as well as in Vertebrata, a close connection is observable 

 between the urinary and the reproductive organs. This is notable in the Annelids, 

 where each chamber of the coelom communicates with the outer medium by means of 

 a more or less coiled tube. These segmental ducts are sometimes enlarged in special 

 regions, so as to carry off the reproductive elements shed into the coelom, thus 

 serving a double purpose. Those anatomists who believe that the ancestral verte- 

 brates were annelid-like base their arguments largely on the very similar conditions 

 which exist in vertebrates, for in these the primitive excretory organs are likewise seg- 

 mentally disposed, and parts derived from them serve for the outward conveyance of 

 the sexual products. 



The essential structure of the kidney, in all vertebrates, is a system of coiled 

 tubules lined with glandular epithelium, derived from the lining membrane of the 

 coelom, with which, in the lower forms, the end of the tubule may communicate. Into 

 each tubule there grows a tuft of finely divided blood-vessels, which interrupt the flow 

 of the blood in such a way as to cause part of its watery constituents to soak through 

 into the cavity of the tubule. Furthermore, the walls of the tubules are richly pro- 

 vided with capillaries, so that the glandular cells can select from the passing blood the 

 effete matters which must be removed from circulation. 



In the fishes and Amphibia the kidney is usually divisible into two parts, — a pro- 

 and a meso-nephros, — in both of which a segmental arrangement is detectable ; but in 

 the higher forms, a third part — the permanent kidney or metanephros — is devel- 

 oped, in which the tubes never show a segmental disjjosition. The duct of the meta- 

 nephros, or ureter, never serves for any other purjDose than carrying off the urine, but 

 such is not the case with the segmental parts of the kidney. In connection with them 

 are formed two ducts, — the Milllerian and the Wolflian, — the former of which is 

 fully developed in the females of all Vertebrates as the oviducts, but is only rudimen- 

 tarily present in the male. The latter has a more complicated fate ; in fishes and 

 Amphibia, the Wolffian duct serves in the female sex merely as a urinary duct, while 

 in the male both the urine and the male reproductive elements are conveyed outwards 

 by it. In the higher forms, the Wolffian duct is barely represented in the female sex, 

 while it serves only as a sperm duct or vas deferens in the male. 



A considerable difference exists between the male and female sexual glands as to 

 their relationshijis to these ducts. The ovaries generally shed the ova into the coelom, 

 whence they are collected by the open mouths of the Mtillerian ducts, but the testes 

 become fnore intimately united with the Wolffian duct by the intervention of certain 

 altered urinary tubules (the epididymis), so that the sperm-cells never fall into the coelom. 



