688 ARTHROPODA. 



There are found in the embryos of Nephelis and Hirudo 

 certain remarkable provisional excretory organs the origin and 

 history of which are not yet fully made out. In Nephelis they 

 appear as one (according to Robin), or (according to Biitschli) 

 as two successive pairs of convoluted tubes on the dorsal side of 

 the embryo, which are stated by the latter author to develop 

 from the scattered mesoblast cells underneath the skin. At 

 their fullest development they extend, according to Robin, from 

 close to the head to near the ventral sucker. Each of them is 

 U-shaped, with the open end of the U forwards, each limb of the 

 U being formed by two tubes united in front. No external 

 opening has been clearly made out. Fiirbringer is inclined from 

 his own researches to believe that they open laterally. They 

 contain a clear fluid. 



In Hirudo, Leuckart has described three similar pairs of 

 organs, the structure of which he has fully elucidated. They 

 are situated in the posterior part of the body, and each of them 

 commences with an enlargement, from which a convoluted tube 

 is continued for some distance backwards; the tube then turns 

 forwards again, and after bending again upon itself opens to the 

 exterior. The anterior part is broken up into a kind of 

 labyrinthic network. 



The provisional excretory organs of the Leeches cannot be 

 identified with the anterior provisional organs of Polygordius 

 and Echiurus. 



Arthropoda. Amongst the Arthropoda Pcripatus is the 

 only form with excretory organs of the type of the segmental 

 excretory organs of the Chaetopoda 1 . 



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

 lateral divisions of the body-cavity, shut off from the main 

 median division of the body-cavity by longitudinal septa of 

 transverse muscles. 



Each fully developed organ consists of three parts : 



(i) 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 



1 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis." 

 Quart. J. of Micr. Science, Vol. Xix. 1879. 



