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 Morphologie 
der Oligochaeten,’ Taf. iv. fig. 2, v.s; and Stolé in SB, béhm. 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 spermathecze (woodcut, fig.1, ¢, p. 441). 
It has been already mentioned that there are no setze developed in 
the neighbourhood of these orifices; the 6th segment, which carries 
the atrial pores, possesses only the dorsal pairs of sete. The 
apertures of the atria are larger than those of the spermathece, and 
rather more conspicuous, for the reason that they are surrounded by 
an area upon which there are ro glandular cells. ‘The atria are 
lined by a columnar epithelium, but I could observe no layer of large 
cells covering these organs externally and forming 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 Sty/aria (Vejdovsky, op. cit. pl. iv. 
fig. 10). 
The vasa deferentia appear to run forwards, and to open by a 
funnel into the 5th segment. 
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 Nais, Ophidonais, 
and Stylaria. In all of these genital sete are found upon the 6th 
segment. Dero differs in the absence of these structures. The 
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. 
