EMBll\OLOGY OF CLEPSINE. 277 



simple groups of mesodermic cells — two in each somatomore, to 

 the right and left of each ganglionic centre. Fig. 91 shows the 

 position of these organs two and a half days after exclusion. 

 Fig. 92 represents one oi these organs one day later. The cells 

 are now arranged in the form of a double-looped string, and 

 have shifted their position between the segment-cells (fig. 91) to 

 a position outside the same. This change of position is due to 

 the centrifugal growth of the mesodermic elements so cha- 

 racteristic of all bilateral animals. 



In fig, 56 I have given one of the segmental organs {nephri- 

 dia, Lankester) in its fully developed form. The string of cells 

 seen in fig. 9£ has lengthened immensely, extending from near 

 the median ventral hne along the floor of the coelom, to the 

 margin, of the body and mounting from here to the median 

 dorsal line. The main body of the organ lies within a single 

 somatomere, but the external orifice (m) is in the ventral floor of 

 the following somatomere. The internal orifice (/«; is formed by 

 a cihated funnel. A duct of very small calibre accompanies the 

 string of cells from end to end. This duct does not pass through 

 the cells, with the exception of the one at the outer extremity 

 {ea), but is apposed to one side of the same. From the duct 

 short lateral branches pass to each cell. Whether these branches 

 have a lumen or not, 1 have not been able to ascertain. 



The duct with its cells passes by a tortuous course from the 

 ciliated funnel towards the margin of the b idy where it becomes 

 labyrmthitorra. Issuing from the labyrinthic coil near the 

 central marginal notch of the gastric diverticulum, it ascends the 

 haemal side of the same a little obliquely backwards, crosses the 

 space separating this from the following diverticulum, and, after 

 reaching the median dorsal line, bends fowards around the 

 interspace of the diverticula and passes down along the posterior 

 side of the diverticulum entering the labyrinth again. It then 

 makes another excursion to the dorsal region and back, in a course 

 parallel at every step with the preceding, thus completing the 

 S-.^haped figure seen in the drawing. A third shorter loop is 

 then added to the 8-figure, after which it passes obliquely 

 backwards to near the centre of the following somatomere and 

 here ends by a vesicular enlargement of the duct [ea). A 

 part of the seventh diverticulum is cut away to show the 

 position of the external orifice. The duct diminishes gradually 

 in calibre from tlie external ventral aperture to the. free ciliated 

 end. There are in the adult worm sixteen pairs of segmental 

 organs, fifteen of which correspond to the fifteen pairs of gastro- 

 pleural caeca. The remaining pair is in the somatomere pre- 

 ceding that which contains the most anterior pair of diverticula. 



(/) Number of Somatomeres.— The original number of body- 



