STCTDIES OX THE ECTOPARASITIG TREMATODES OF JAPAX. 135 



genital .atrium should be regarded as the invagination of a portion of 

 the surface of the body. We have as yet no adequate enibryological 

 proof"' on this point ; but the fact that the investing membrane of 

 the body is in many cases directly continued, at least for a certain 

 distance, into the atrium, and the many gradations in form of 

 this chamber from that of a mere pit to that of a deep and spacious 

 cavity seem to me to lend very strong support to this view — :a view 

 which has recentl}' found expression from another side^' 



The genital atrium is most developed and presents the most 

 varied forms in Microcotijlc. In this genus it is a deep cave-like cavity, 

 into which the vas deferens and the uterus open, and communicates 

 with the exterior by means of a minute opening, the forus fjcnitaUs com- 

 munis. Its inner surface is lined by a thin but deeply staining mem- 

 brane which in sections is seen to be formed by the direct continuation 

 and gradual thinning of the investing membrane of the body (Pis. Y, 

 & VI), and is covei-ed in all the species with hollow, chitinous spines. 

 In M. clegans (PI. Y, fig. 2.) it is a somewhat reniform cavity, with 

 the part corresponding to the hilus directed obliquely forwards and 

 towards the ventral side, into which the uterus opens close to the ex- 

 ternal pore, while the vas deferens opens more on the dorsal side almost 

 opposite the hilus. The conical spines which are mainly confined to 



1). The results obtained by Voeltzkow in Aspidogaster oonchicola (Wiirzburger Arbei- 

 ten, Bel. 8, 188S, p. 279) do not seem to me to decide the question. According to this naturalist 

 the " Penisschlaueh " and the "Vulva" arise "als solider, von der Haut am Anfang des 



Halses, sohief nach hinten uud oben aufsteigender Zapfen an." Eeierring to the figure it 



seems to me that the solid mass of cells which becomes afterwards the " Penisschlaueh " and 

 the " Vulva " is not of an ectodermal origin but mesodermal ; for the true ectoderm is already 

 in this stage transformed into a membrane. If I may put my own interpretation on the figure 

 of another, the solid mass 01 cells above referred to is probably differentiated into the peculiar 

 connective tissue that constitutes the penis ; and in this case the lining of the " Penissohlanch" 

 would alone be derived from the ectoderm. 



2). MonticeUi— Primo contributo di osservazioni sui Distomidi (Studii sui Trematodi endo- 

 parassiti), p. 86. Spengel's Zool. Jahrbucher, III. Suppl,, 1893, 



