EXCRETORY ORGANS. 689 



and at the other, as I believe, into the body cavity. This 

 section becomes very conspicuous, in stained preparations, . by 

 the intensity with which the nuclei of its walls absorb the 

 colouring matter. 



In the majority of the Tracheata the excretory organs have 

 the form of the so-called Malpighian tubes, which always (vide 

 Vol. II.) originate as a pair of outgrowths of the epiblastic 

 proctodaeum. From their mode of development they admit of 

 comparison with the anal vesicles of the Gephyrea, though in 

 the present state of our knowledge this comparison must be 

 regarded as somewhat hypothetical. 



The antennary and shell-glands of the Crustacea, and 

 possibly also the so-called dorsal organ of various Crustacean 

 larvae appear to be excretory, and the two former have been 

 regarded by Claus and Grobben as belonging to the same 

 system as the segmental excretory tubes of the Chaetopoda. 



Nematoda. Paired excretory tubes, running for the whole 

 length of the- body in the so-called lateral line, and opening in 

 front by a common ventral pore, are present in the Nematoda. 

 They do not appear to communicate with the body cavity, and 

 their development has not been studied. 



Very little is known with reference either to the structure or 

 development of excretory organs in the Echinodermata and the 

 other Invertebrate types of which no mention has been so far 

 made in this Chapter. 



Excretory organs and generative ducts of the Craniata. 



Although it would be convenient to separate, if possible, the 

 history of the excretory organs from that of the generative 

 ducts, yet these parts are so closely related in the Vertebrata, in 

 some cases the same duct having at once a generative and a 

 urinary function, that it is not possible to do so. 



The excretory organs of the Vertebrata consist of three 

 distinct glandular bodies and of their ducts. These are (i) a 

 small glandular body, usually with one or more ciliated funnels 

 opening into the body cavity, near the opening of which there 

 projects into the body cavity a vascular glomcrulus. It is 

 situated very far forwards, and is usually known as the head- 



44. 



