POLYSTOMID^. 48 



to the general trematode type, exhibiting only certain departures 

 in their mode of arrangement. All are hermaphroditic, the eggs 

 being supplied with filamentary appendages, in some only at one 

 pole of the shell, in others at both ends. The water-vascular 

 system is conspicuously developed. All the species are supplied 

 with an armature of prehensile hooks, these homy structures being 

 either connected with their complicated series of suckers, or else 

 placed altogether independently apart, in which case they are 

 usually situated in the central line of the body. In the sexually 

 immature Diporpa-condition of the genus Diplozoon, there are two 

 supernumerary hooks associated with a dorsal sucker at the centre 

 of the body, and it is by means of these anomalously-placed organs 

 that a conjugation between two such juvenile forms is effected. 

 These two individuals become organically united for life, after the 

 fashion of the Siamese twins ; and, according to the investigations 

 of Yon Siebold, it is not until after this conjugation has been con- 

 summated that the sexual organs make their appearance. In 

 Oncliotyle apjpendiculata we are presented with peculiarities of form 

 scarcely less remarkable. In this creature the lower end of the 

 body suddenly merges into a curious appendage which is placed 

 almost at a right angle with the body itself, and in this way, as 

 Van Beneden justly remarks, the entire animal resembles a little 

 hammer ; the resemblance being very much heightened by the 

 circumstance that one end of the appendage is cleft so as to cor- 

 respond, as it were, with the notch which we employ in the action 

 of nail-drawing. At the bifurcate extremity of this singular 

 tail Van Beneden describes the occurrence of two oval contractile 

 vesicles. These organs open separately, one at either point, being 

 also internally connected with the water-vascular system, which is 

 consequently double in this genus. In point of function. Van Bene- 

 den regards these vessels as uriniferous. A little above the pulsatile 

 vesicles we find two recurved hooks, which are likewise bifurcate 

 at their base, and there are other prehensile elements occurring in 

 connection with these suckers. Each one of these last-named 



