388 ADAM SEDGWICK. 



Somites of Legs 13 to 17 (Part III, p. 510).— The 

 dorsal divisions of these somites persist as the generative 

 organs, and will be described below (PL XXIX, diagrams 

 figs. 15 — 17). The ventral divisions develop as in the legs 

 immediately preceding. 



Somites of the Anal Papilla (or in P. Balfouri of the 

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

 on p. 513 of Part III. They persist entirely as parts of the 

 generative ducts. For descriptions and figures of the isolated 

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

 reader to Balfour's memoir (No. 2), pp. 32 — 35, and PL 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 

 ccelomic, but, as shown in PL XXVII, 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 PL XXVII, 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 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 in Part III, pp. 511 — 515, and I have but little to add 

 to that description. They first appear in the endoderm as large 

 round nuclei (Part III, figs. 26, 27), which migrate into the 

 splanchnic mesoderm (Part III, fig. 41) of the dorsal divisions of 



