DISTOMA HEPATICUM. 259 



the opposite wall expanded, or vice versa. Sometimes this move- 

 ment proceeded from behind forwards, sometimes from before 

 backwards. Anteriorly the ductus ejaculatorius opens into a 

 membranous, sickle-shaped, very thick penis, perforated through 

 the middle, beset, especially towards the apex, with concentric, 

 close layers of very distinct points or spines, which generally fall 

 off easily. The spinous external skin of the penis is an inversion 

 of the general cutaneous covering, of which a fine layer also 

 enters into the inner opening of the penis. When the back of 

 an ordinarily fine scalpel is placed perpendicularly upon the 

 anterior margin of the ventral sucker, and pressure is applied 

 with a second scalpel directed obliquely from before backwards, 

 from the buccal sucker towards the first scalpel, it is always easy 

 to cause the protrusion of the penis. If it be then cut away at 

 the ventral surface of the animal with a pair of scissors, we shall 

 easily detect the coating of spines, which, however, appears to be 

 very easily lost. In this way every one may readily convince 

 himself of the existence of the spines. This coating of spines 

 appears to me to furnish an argument in favour of the statement 

 of some authors, as ; for instance, Dujardin, that the epidermis of 

 Dist. heputicurn is beset throughout with spines. As this spinous 

 coat of the skin certainly always occurs normally, and pretty 

 evenly diffused in the earliest portion of the life of our Distomum, 

 but in course of time is lost by the movements of the animal, 

 without leaving any traces, this may also probably be the case 

 with the penis ; and it is only the rare use of this organ, and its 

 inverted position, that preserves it from having its spines rubbed 

 off. Thus the spines of the penis remain, as it were, the last 

 traces of the previous very general spinous coating of the epider- 

 mis. As regards the act of protrusion of the penis itself, it is 

 most probable that this takes place voluntarily in the Distoma in 

 the following manner. The parenchyma of the animal in the 

 vicinity of the penis, or, as Mehlis thinks, the membrane enve- 

 loping the anterior sexual apparatus (i. e., the penis-sac), con- 

 tracts circularly, and thus presses from the sides upon the root of 

 the penis, whilst at the same time the skin is undoubtedly 

 in a certain state of expansion, but not of contraction. This 

 appears from the simple consideration that the root of the penis 

 is pretty thick, and measures at least i'" in diameter. The 

 opening from which the penis protrudes through the skin is not 

 usually perceptible, or, at the utmost, forms a cleft-like inversion, 



