252 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



other directly to the exterior, recall the characters of 

 the segmental organs of the earthworm ; there may 

 be one or a few pairs of these tubes, and their excretory 

 nature .is assumed from the presence in them of a 

 brown concretion (as in the so-called " brown tubes " 

 of Sipunculus) ; in certain forms they do, without 

 doubt, lose their excretory, and take on the function 

 of efferent ducts for the generative products, an 

 arrangement which is by no means confined to the 

 Gephyrea among animals ; in Bonellia, the tube which 

 functions as the uterus is developed on one side only 

 of the body. 



It is of especial interest to observe that in the 

 developing leech three pairs of canals are developed 

 in the hinder end of the body, and are, at least, pro- 

 visional excretory organs, even if they are in no way 

 related to the cloacal outgrowths of lower worms. 

 The permanent nephridia of the leech attain to a 

 very high degree of complexity ; it is possible to 

 distinguish a vesicle and a gland, connected with 

 one another by a vesicle duct (Bourne). The cells 

 of the gland are all penetrated by ductules, and the 

 central portion of each of its four constituent lobes is 

 occupied by a duct which opens into the vesicle 

 duct ; a plexus of blood-vessels is found in the 

 gland, each cell of which is surrounded by a loop of 

 that plexus ; the wall of the vesicle is muscular, and 

 by its contractions the contents are expelled to the 

 exterior. The marine ecto-parasitic leech Pontob- 

 della is remarkable for the possession of a very 

 primitive disposition of the nephridia ; the organ is 

 single and continuous, and consists of a highly com- 

 plicated network of tubules ; those on one side of the 

 body are continuous with those of the other, and with- 

 out developing any vesicle, they open to the exterior 

 at regular intervals (Bourne). 



In the lower Crustacea an excretory function is 



