272 
The Sexual Form of Ch^togaster Limnjei. By E. Ray 
Lankester, B.A. Oxon. With Plates XIV and XY. 
In vol. xxvi of the e Transactions of the Linnean Society ’ 
is published a paper by me, read in December, 1867, de- 
scribing, amongst other matters, the general structure of the . 
rapidly-reproducing fissiparous worms found abundantly on 
Limnseus and Planorbis. At the time when that paper was 
read I had not succeeded in obtaining these worms in a 
sexually mature condition, though, as mentioned in a note, I 
was fortunate enough to do so while the paper was passing 
through the press. For more than three years I have taken 
every opportunity of examining the little worms on the water- 
snails ; and it was only last October, during the first week of 
that month, that I was gratified with the sight of individuals 
possessing sexual reproductive organs. They were only to 
be obtained for the space of one short fortnight, and then 
disappeared. The sexual individuals were larger than the 
fissiparous forms, which continued to abound, the former 
being comparatively rare — about in the proportion of one 
sexual individual to twenty of the chains of fissiparous zooids. 
The mature worms, which thus so transiently and sparsely 
make their appearance, present certain very interesting modi- 
fications of structure as compared with the immature zooids, 
which I shall endeavour to point out, though I may here 
remark that I look forward to a further examination next 
October, to clear up some matters of doubt ; the arrangement 
of the reproductive organs themselves is also a subject of 
much importance in these worms, and has not hitherto been 
satisfactorily stated. 
The only accounts of the reproductive organs of Chseto- 
gaster with which I am acquainted are to be found in two 
papers published by M. Jules d’Udekem, a naturalist by 
whose early death this branch of zoology has lost a per- 
severing and able investigator. In his valuable memoir on 
the reproductive organs and development of the Earthworm, 
published in the ‘ Mem. des Savants Etrang.’ of the Academy 
of Belgium, 1856, vol. xxvii, INI. d’Udekem gives a diagram- 
matic sketch of the genitalia of Chcetogaster diaphanus. In 
the c Bulletins’ of the same Academy, vol. xii, 1861, p. 243, he 
gives another sketch, “ very schematic,” as Professor Leydig 
observes, of the genitalia of Chcetogaster Mulleri, accom- 
panied with a brief explanation, and also a description of 
