M. E. Mecznikow on the Rhabdocoela. 57 



XI. — On the Rhabdocoela. By E. Mecznikow*. 



[Plate VIII.] 



In his great work on the anatomy and developmental history of 

 the lower marine animals, Claparede has expressed the opinionf 

 that the Rhabdoccela must be divided into two groups corre- 

 sponding with the two divisions of the Dendroccela. 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 Rhabdocoelan 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 J and Max Schultze§. In 

 this animal the unequal development of the male and female 

 organs in difi"erent individuals appears most remarkable : some- 

 times we meet with those which exhibit an aborted female ap- 

 paratus along with a fully-developed male (PI. VIII. fig. 1) or 

 vice versa (fig. 2). In the former we find a large unpaired 

 testis (fig. 1 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.r.) ; moreover in these individuals there 

 is an isolated round vesicle, or receptaculum seminis, containing 

 granules (r. 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. 2ov.), a simple gland 



* Translated by W. S. Dallas, F.L.S., from Wiegmann's 'Archiv,' 1865, 

 pp. 174-181. 



t Beobachtungen iiber Anatomic und Entwickelungsgeschichte wir- 

 belloser Thiere, 1863, p. 16. 



X Die Rhabdocoelen Strudelwiirmer, 1848, p. 26. 



§ In Carus's Icones Zootomicae, tab. 8. fig. 16. 



