140 CHANGES FROM STAGE O TO BIRTH. 



be described below (PI. XIIT, diagrams figs. 15 — 17). The 

 ventral divisions develop as in the legs immediately preceding. 



Somites of the Anal Papillae (or in P. Balfour i of the 

 eighteenth legs). — I have nothing to add to the description 

 given on p. 95. They persist entirely as parts of the gene- 

 rative ducts. For descriptions and figures of the isolated 

 nephridia ol the seventeen legs of the adult I must refer the 

 reader to Balfour's memoir (No. 20), pp. 32—35, and PI. XIX, 

 figs. 27, 28. I have nothing to add to his description, except- 

 ing the fact that the terminal portions of the nephridia do not 

 open into the body cavity, which is a vascular space and not 

 eoelomic, but, as shown in PI. XI, fig. 11, and in diagram 

 fig. 17, into a thin-walled vesicle, which is directly derived 

 from the original somite. 



I think there can be no doubt that the vesicle of the 

 nephridia of the first three legs is homologous with the 

 internal vesicles of the posterior nephridia and not with the 

 collecting, or external vesicle. A comparison of figs. 10 and 

 11 on PI. XI, shows that the tubular part of the first three 

 nephridia is very different from the narrow tube leading outward 

 from the external vesicle in the posterior nephridia; though it 

 is without the closely-packed nuclei in the terminal so-called 

 funnel. Further, the structure of the wall of the vesicle itself 

 resembles that of the internal vesicle of the posterior nephridia 

 and not that of the collecting (external) vesicles. 



The external cuticle is only prolonged for a very short 

 distance into the neck of the collecting vesicle. 



The Generative Organs. 



The early history of these organs has already been fully 

 described on pp. 93 — 98, and I have but little to add to 

 that description. They first appear in the endoderm as large 

 round nuclei (PI. VII, figs. 26, 27), which migrate into the 

 splanchnic mesoderm (PL VIII, fig. 41) of the dorsal divisions of 

 the sixteenth to the twentieth somites, where they acquire a pro- 

 toplasmic investment (Pi. IX, figs. 43, 47). The parts of the 



