ON TUBIFEX. 153 



previous to which, however, they have ruptured the vitelline mem- 

 brane in which each is enclosed (see Fig. lo), and may be seen 

 actively moving about within the capsule, seeking for an exit which 

 they at length effect by breaking off one of the soft projecting 

 poles (see Fig. ii). They are then about one-eighth of an inch 

 long (see Fig. 13), and of a white colour ; the alimentary canal, 

 which at this time occupies nearly all the space within the body, 

 being filled with yelk spherules, a provision apparently for their 

 sustenance till sufficiently grown to take care of themselves. The 

 vascular system at this time is difficult to detect, not, I think, that 

 the vessels do not exist, but from the absence of colour, as yet, 

 in their contents. The pulsating hearts, however, in the eighth 

 (seventh setegerous) segment have a faint tinge. The hooked 

 setae are present ; one or two only in a fascicle ; but the dorsal 

 capillary setae are slow in making their appearance, and cannot be 

 detected till the fourth or fifth day after birth. The young worms 

 at first consist of about thirty segments. Others are subsequently 

 added by the subdivision of the last segment. There are no 

 indications of the reproductive organs, for the segments in which 

 they are subsequently found present no characters to distinguish 

 them from the others. 



I have recently been somewhat interested by the occurrence in 

 these worms of a parasite, Gregariiia scenuridis, in an encysted 

 condition in the matrix with the ova, and in the segments anterior 

 to this. On the first occasion of finding them I was greatly 

 puzzled with them, thinking I had found some peculiar con- 

 dition of development in the ova of the worm. I soon, how- 

 ever, recognised the characteristic pseudo-naviculae in the cysts, and 

 a reference to Gegenbaur at once solved the difficulty. The cysts 

 of this parasite are always found in the matrix, together with the 

 ova; their presence there is, however, destructive to the latter, which 

 either undergo atrophy, or do not come to maturity. A few remains 

 of yelk may sometimes be seen, but never fully-matured eggs, the 

 nutritive juices which should have nourished them being apparently 

 used up for the development of the parasite. A drawing of the 

 cysts is given in two different conditions in Figs. 6 and 7, and of 

 the parasite itself in the act of conjugation from Gegenbaur in Fig. 

 9. The following extract ^ will explain the matter : — " The mode 

 * Gegenbaur's Comp, Anat., Bell's Translation, p. 87. 



