1903. ] POLYCLADS OF THE “‘SKEAT EXPEDITION.” 307 
the point where the uteri open into it, the muscular walls, which 
are elsewhere feebly developed along its course, become thicker, 
but not very markedly so (cf. LZ. californica von Plehn). The 
two uteri (wé.) unite as they enter it from below, and behind them 
there is a very small accessory vesicle (ace.ves.) with muscular 
walls. 
The genus Leptoplana, as at present constituted, contains a 
considerable number of species, many of them only referred by 
Lang with doubt to the genus. In the majority of the species, 
the anatomy of which has been investigated by means of serial 
sections, the genital apparatus presents the following characters :— 
The penis is directed backwards. The vasa deferentia open 
into a muscular vesicula seminalis ; from this the ductus ejacula- 
torius runs backwards to open into a prostate organ, which may be 
chambered ; leaving this, the duct runs back into the penis, 
which may be armed with a chitinous stylet. 
The vagina runs back from the antrum femininum, receives the 
secretion of the shell-glands, and further back the two uteri open 
into it ventrally, usually by a common duct. An accessory vesicle 
is generally present. The most familiar species of the genus, Z. 
tremellaris, differs from the majority of species in the structure of 
its male apparatus sufficiently to permit us to put it on one side 
to form of itself a section of the genus further characterised by 
the possession of a ventral sucker between the genital openings. 
Another species, Z. swbviridis von Plehn [8] (=Z. pardalis mihi 
[4]), approaches Discocelis tigrina in the structure of its female 
organs, and accordingly we may put this species also in a section 
apart. 
The remaining species I have attempted to classify below, so far 
as is at present possible, according to the structure of the penis 
and prostate. I trust that I will not be thought to have laid too 
much stress on the structure of these organs in dealing with this or 
other genera. It seems to me that, of all the characters that 
present themselves for classifying this order of Turbellaria, these 
are the most useful, and that they are as reliable as any of the 
other characters employed for this purpose, such as structure of 
the pharynx, number and arrangement of eye-spots, presence or 
absence of tentacles, &e. &e. 
I propose, then, to group the species in two sections, A, and B, 
so far as our knowledge of their anatomy permits: 
A. Penis provided with a stylet. 
(a) Prostate complicated by radially arranged ‘“ Drusen- 
kaniile.” 
1. L. kukenthali v. Plehn. Spitzbergen [7]. 
(6) Prostate divided into chambers lying parallel to the 
ductus ejaculatorius. 
L. vitrea Lang. Mediterranean ret 
wo bo 
. L. aleinoi Schmidt. 6]. 
9 
