M. EB. Mecznikow on the Rhabdocecela. 57 
XI.—On the Rhabdocela. By E. Mecznixow*. 
[Plate VIL. ] 
In his great work on the anatomy and developmental history of 
the lower marine animals, Claparéde has expressed the opiniont 
that the Rhabdocela must be divided into two groups corre- 
sponding with the two divisions of the Dendrocela. He founds 
this opinion upon the fact that the genera Convoluta and Macro- 
stomum possess two genital orifices. Although I can confirm 
this observation from my own investigations, and even add a 
third Rhabdoccelan with two genital apertures to those just 
mentioned, I must affirm that this peculiarity of the organs 
of generation, from its irregularity, cannot furnish any classifi- 
catory character either for the chief divisions or even for the 
genera. The following statements as to the sexual organs of 
some species of Prostomum may serve as a proof of this. 
I will first call attention to the common freshwater form, 
Prostomum lineare, the sexual organs of which have already 
been investigated by Oscar Schmidt t and Max Schultze§. In 
this animal the unequal development of the male and female 
organs in different individuals appears most remarkable: some- 
times we meet with those which exhibit an aborted female ap- 
paratus along with a fully-developed male (Pl. VIII. fig. 1) or 
vice versd (fic. 2). In the former we find a large unpaired 
testis (fig. 1 ¢), which communicates with a vesicle containing 
seminal masses (v.s.); this opens into another thick-walled ve- 
sicle, in which the zoospermia are converted into a compact 
mass. After this vesicle has received several currents of fatty 
corpuscles (c. ad.), which are evidently related in some way to 
the zoospermia, it is connected with the spinous apparatus which 
acts as the penis. In the individuals just described we find no 
poison-gland, and only few traces of the female organs, namely 
some isolated ovicells (o.7.) ; moreover in these individuals there 
is an isolated round vesicle, or receptaculum seminis, containing 
granules (7. s.). 
In the other individuals of Prostomum lineare the male organs 
are in a rudimentary state, as the testis alone can be detected in 
them, whilst the two seminal vesicles have disappeared entirely. 
The female organs of such individuals, on the contrary, are 
completely developed. The ovary (fig. 2 ov.), a simple gland 
* Translated by W.S. Dallas, F.L.S., from Wiegmann’s ‘Archiv,’ 1865, 
pp- 174-181. 
+ Beobachtungen iiber Anatomie und Entwickelungsgeschichte wir- 
belloser Thiere, 1863, p. 16. 
{ Die Rhabdoccelen Strudelwiirmer, 1848, p. 26. 
§ In Carus’s Icones Zootomice, tab. 8. fig. 16. 
