OCR Text |
Show 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 Stole 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 spermathecae (woodcut, fig. 1, J, p. 441). It has been already mentioned that there are no setae developed in the neighbourhood of these orifices; the 6th segment, which carries the atrial pores, possesses only the dorsal pairs of setse. The apertures of the atria are larger than those of the spermathecae, 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 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 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. It is clear therefore from the brief and, in some respects, incomplete 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 setae are found upon the 6th segment. Dero differs in the absence of these structures. The form of the atrium is more like that of Nais 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. |