STUDIES ON THE ECTOPAEASITIC TREMATODES OP JAPAN. 141 



the atrium; so that here the atrium is locally enlarged and sends out 

 a lateral evagiuation on one side (PI. XXVI, fig. 3). In both the 

 species of Epibdella above mentioned the genital atrium extends up to 

 the very base of the penis, so that this projects wholly into it. In 

 Tristomum, on the other hand, the basal portion of the penis lies imbed- 

 ded in the mesenchyma, so that the genital atrium forms a sheath only 

 for the terminal portion of the penis. Its form is very varied 

 in different species; but always consists of a more or less tubular distal 

 portion of various length and a more enlarged proximal portion 

 forming the sheath for the penis. The internal surface is lined by a 

 thin membrane, which is the direct continuation of the external mem- 

 brane of the body on one side and of the investing membrane of the 

 penis on the other. Where the uterus opens into the genital atrium, 

 its opening lies at various distances from the external atrial pore 

 in different species, being in some situated close to it, while in others 

 it is far removed towards the bottom of the atrium (PI. XXI, fig. 8; 

 PI. XXII, figs. 2 & 4; PI. XXIII, fig. 8; PL XXV, figs. 3, 

 8, & 9). 



The genital atrium of Tristomum and Epibdella is wholly destitute 

 of any chitinous armature. 



In most species of Axine and Microcotijle the conical spines that 

 arni the genital atrium are distinctly seen to be formed by the eleva- 

 tion and transformation of the lining, membrane (PI. V, fig. 3; PL VI, 

 fi^. 1; also in figs. 1 & 2, PL V, though not clearly shown in the 

 figures). These spines are, as already stated, hollow, and the internal 

 cavity is filled in most species with a deeply staining, homogeneous 

 substance which is directly continuous with the basement membrane 

 and is indistinguishable from it by. optical characters. In M. chiri 

 (PL V. fig. 4), M. reticulata (PL V, fig. 6), and M. sciaenae (PL VI, 

 fig. 2) the cavity in the spines is reduced to very small dimensions 



