260 ANIMAL PARASITES. 



but, as may be seen from the diameter of the penis, it is capable 

 of considerable dilatation. According to Aubert, there are 

 two muscular layers in the penis of other large Trematoda 

 " Longitudinal, elastic, circular, and, perhaps, transverse fibres, 

 although we cannot speak of a true transverse striation of 

 the muscles, as the substance, when crushed, loses its striation 

 and breaks up into irregular fragments/' It is the same in this 

 case also. 



I will also mention here a simple method by which the portion 

 of the sexual organs of which* I have last spoken may be best 

 isolated. From the firm consistence of these parts, we may 

 easily succeed, with a fine needle, in laying them bare and 

 isolating them entirely. It was in. a penis-sac, with its ap- 

 pendages, removed in this way from the living animal, that I 

 saw the before-mentioned peristaltic movement of the ductus ejac- 

 ulatorius most beautifully. The relative positions of the external 

 opening of the retracted penis and the vagina, are as follows, as cor- 

 rectly described by Mehlis : Near the anterior margin of the ventral 

 sucker we observe a small oblong, or obtusely triangular, pit or 

 cleft. If this be drawn asunder, two openings make their 

 appearance, of which that situated posteriorly, and towards the 

 left, is the vaginal orifice, and the other that of the penis. 



If we glance once more at the sexual conditions of our Dis- 

 tomum, we find, with the compound microscope, in the interior of 

 the hindermost convolutions of the uterus, reddish-brown aggre- 

 gations or masses of spermatozoa, which exhibit the most lively 

 swarming on their free margins, and may be isolated as moderate 

 agglomerations. To the naked eye these spots appear white ; 

 they project posteriorly beyond the level of the other convolu- 

 tions of the uterus, and in attempting to isolate these white 

 spots, we are sure, with some practice, to find immense numbers 

 of aggregated spermatozoa. As some of these formations appear 

 to lie in the uninterrupted canal of the uterine convolutions, 

 we may perhaps assume that they have reached this spot in 

 consequence of a self-impregnation accompanied by copulation ; 

 but, on the other hand, as the testicle undoubtedly also opens 

 here directly towards the true dilatations of the uterus, 1 they 

 may be the consequence of a self-impregnation without copula- 

 tion. The Distoma are therefore hermaphrodites, with the fol- 



1 Recently I succeeded in pressing spermatozoa out of the testicles through the 

 funiculus spermaticus into the sac of the penis, under the microscope. 



