444 ON THE NATURAL HISTORY OF DERO. [Nov. 5, 



On the other hand, the sperm-sac and egg-sac were filled with the 

 sexual products. 



The sperm-sac is a single unpaired structure extending through 

 segments 6-8 ; it was filled with spermatozoa and furnished, as is 

 usual in the Naidomorpha (see Vejdovsky, ' System und Morphologic 

 der Oligochaeten, ' Taf. iv. fig. 2, v.s ; and Stoic in SB. bohm. Ak. 

 1887, p. 143), with a pair of rhythmically contractile vascular trunks. 



The median unpaired egg-sac lies behind the sperm-sac, occupying 

 segments 8-10; it contained 2-4 large ova with abundant yolk- 

 spherules. The diameter of the largest ova was fully that of the 

 body-cavity. 



The atria open on to the ventral surface of the body in a line 

 with the openings of the spermathecse (woodcut, fig. 1, c?, p. 441). 



It has been already mentioned that there are no setse developed in 

 the neighbourhood of these orifices ; the 6th segment, which carries 

 the atrial pores, possesses only the dorsal pairs of setge. The 

 apertures of the atria are larger than those of the spermathecse, and 

 rather more conspicuous, for the reason that they are surrounded by 

 an area upon which there are no glandular cells. The atria are 

 lined by a columnar epithelium, but 1 could observe no layer of large 

 cells covering these organs externally and foruiing the structure 

 which has been sometimes termed prostate. No doubt a fine layer 

 of peritoneal cells is present ; but this layer was not conspicuously 

 developed as it is, for example, in Stylaria (Vejdovsky, op. cit. pi. iv. 

 fig. 10). 



The vasa deferentia appear to run forwards, and to open by a 

 funnel into the .5th segment. 



"o 



It is clear therefore from the brief and, in some respects, incom- 

 plete account which I am able to give here of the reproductive 

 organs of Dero, that this genus agrees in all essentials with other 

 Naidomorpha. There is no longer any room for doubt that it has 

 been correctly referred to this family. The three other genera in 

 which the sexual organs have been described are Nats, Ophidonais, 

 and Stylaria. In all of these genital setee are found upon the 6th 

 segment. Dero differs in the absence of these structures. Tlie 

 form of the atrium is more like that of Nats than Stylaria ; it tapers 

 off gradually into the vas deferens, while in Stylaria there is an 

 abrupt line of demarcation between atrium and vas deferens. 



