i6o Elementary Zoology. 



system (e.g. pronephric duct) serve in the adult as gonad ducts. 

 It cannot, however, be argued on d, priori grounds that this is 

 necessarily the case with other animals. And yet there are 

 certain facts which seem to indicate that, generally, there is 

 a connection between nephridia and gonad ducts. In the 

 earthworm there are certain obvious similarities which cannot 

 be passed over. Genital ducts, as well as nephridia, open by 

 open ciliated mouths into the coelom, and by a pore on to the 

 exterior. In the case of the oviducts the whole tube occupies 

 two segments, perforating the septum as does a nephridium. 

 The male ducts, it is true, are so far unliice nephridia that they 

 traverse several segments on their way to the exterior; but 

 there are many worms belonging to the same large group as 

 that which contain the earthworm in which the male ducts also 

 occupy but two segments. There are some other facts which 

 point in the same direction, but we shall not enter into 

 a description of them here, as they would be strengthened by 

 further investigation. With regard to the anodon, the genital 

 ducts are not only distinct from the nephridia, but they open 

 on to the exterior by a separate pore ; but among the LameUi- 

 branchiate Mollusca there are forms in which the genital duct 

 opens by the same pore, and others in which the two ducts are 

 a common tube for some distance ; this looks much as if the 

 gonad duct is really a part of the nephridium split off. The 

 genital ducts of the crayfish have this fact in common with the 

 green glands (hypothetically, at any rate, nephridia), that they 

 open on to the thoracic appendages at a point precisely corre- 

 sponding with the place of opening upon the antennse of the 

 green glands. There is a suggestion here of a series of 

 metamerically arranged nephridia, of which only one — the 

 green gland — has retained its excretory function, while two 

 others remain, one in each sex, as either oviduct or sperm duct. 



Vascular System. 



The vascular system is not found in the simpler coelomate 

 animals. It consists essentially of a system of spaces, con- 

 taining a fluid, and excavated in the mesoblast. This system 



