508 VERTEBRATA. 



These organs are placed at the bases of the feet, in the lateral 

 divisions of the body cavity, sbut off from the main median division 

 of the body cavity by longitudinal septa of transverse musc!es. 



Each fully developed organ consists of three parts : 



(1) A dilated vesicle opening externally at the base of a foot. 

 (2) A coiled glandular tube connected with this, and subdivided again 

 into several minor divisions. (3) A short terminal portion opening 

 at one extremity into the coiled tube and at the other, as I be- 

 lieve, 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. i.) 

 originate as a pair of outgrowths of the epiblastic proctodeum. From 

 their mode of development they admit of comparison with the anal 

 vesicles of the Gephyrea, though in the present state of our know- 

 ledge 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 (>laus and 

 Grobben as belonging to the same system as the segmental excretory 

 tubes of the Chabtopoda. 



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 

 liistory 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 (1) 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 glomerulus. It is situated very far forwards, and is usually 

 known as the head- kidney, though it may perhaps be more suitably 

 called, adopting Lankester's nomenclature, the pronephros. Its duct, 

 which forms the basis for the generative and urinary ducts, will be 

 called the segmental duct. 



