156 Elementary Zoology. 



traced. Not in the adult so clearly as in the tadpole. The 

 details have been, to some extent, dealt with in the preced- 

 ing pages upon embryology. The general facts that can be 

 deduced from those details are that in the frog and other 

 vertebrates there are a series of paired excretory tubules, 

 opening into the ccelom, on the one hand, and on to the 

 exterior, on the other, by means of a continuous longitudinal 

 duct. These differences do not, however, invalidate the 

 comparison, for there are worms in which some, at any rate, 

 of the paired nephridia are connected by a longitudinal duct ; 

 while another fact about the segmental tubes of the Vertebrata — 

 that they are not strictly metamerically arranged — is paralleled 

 in other worms. 



The essential likeness remains, that in vertebrates no less 

 than in annelids, there are a series of tubes which open into 

 the ccelom, on the one hand, and on to the exterior, on the 

 other, and that these tubes in both cases are the organs for the 

 excretion of waste nitrogenous products. 



The excretory organs of Asiaats and of Periplaneta are not 

 so easily to be referred to the same series. 



They are both animals with but little ccelom. The 

 excretory organ of Astacus is so far like that of Lumbricus 

 that it is a tube which is partly glandular, and which opens 

 on to the exterior by a non-glandular portion. Of this non- 

 glandular portion there is an appended sac — a diverticulum. 

 In describing and figuring that organ attention has been 

 directed to the fact that the glandular part of the organ 

 terminates in a little oval sac — the terminal sac. It is held 

 by some that this is a small pocket of coelom; and in this 

 case the excretory organ of Astacus — the green gland, as it is 

 usually termed — will agree in essentials with a nephridium of 

 Lumbricus. As for the large vesicle, which is a diverticulum 

 of the duct of the nephridium, it presents no difficulties when 

 compared with the nephridium of worms, for in many species of 

 annelids the duct is provided with an appendix essentially similar. 



There are, however, other arguments which may be 

 advanced to show the correspondence of the green gland 

 of Astacus with the nephridium of an earthworm. In the 



