THE NEPHRIDIA. 



205 



of the nephridium being continued into the terminal saccular 



enlargement (Fig. 102, es). 



We must thus assume that the nephrostome of the Annelida is represented 

 by the opening of the nephridium into the terminal sac. The terminal sac, 

 therefore, corresponds to the coelom (secondary body-cavity of the Annelida), 

 a view which is confirmed by the manner in which the nephridia arise. A part 

 of the coelom has thus come into direct relation to the kidney, and a state of 

 things is found very similar to what has already been met with in the Crustacea 

 (Vol. ii., p. 180), and, with certain modifications, will be found to recur in the 

 Mollusca. 



fc. 



me 



Fig. 102.— Transverse section through the posterior region of the body of an advanced embryo 

 of P. capensis (after Sedgwick, somewhat diagrammatic), e, rudiments of the crural glands ; 

 d, intestine; en, end-sacs of the nephridia; h, heart; l.lh, lateral, m.lh, median, p.lh, pedal 

 portions of the adult body-cavity ; Hies, mesodermal tissue ; n, ventral nerve-trunk ; tie, 

 nephridial canal ; oc, external aperture of the nephridium ; p, foot ; pe, pericardial cavity ; 

 ps, pericardial septum ; sb, collecting vesicle (urinary bladder) of the nephridium ; sd, slime- 

 gland ; so, sole of the foot (thickening of the ectoderm) ; st, transverse commissure 

 connecting the nerve-trunks (n) and the ventral organ (w) ; g, gonad. 



The above description of the simple formation of the nephridia 

 applies specially to those of the segments carrying the first to third 

 limbs (of P. capensi*). Those of the following limbs are distinguished 

 by the fact that the canal becomes much coiled in later stages and 

 widens towards its outer end (Sedgwick, Fig. 102, sb), like the urinary 

 bladder in the nephridia (antennal glands) of the Malacostraca. 



Apart from the transformation which we shall find in the nephridia during 

 the formation of the salivary glands and the genital organs, there air im- 



