20 INTRODUCTION. 



waters in which the Cer caries live. I can, however, offer a 

 solution of this apparent mystery, having surprised many Cercarics 

 in the act of migrating. Before I say anything more about 

 this, I must mention a peculiarity which is to be noticed 

 in most of the Cercaria after they have left their sacs. This is 

 their habit of encysting themselves, a process which is effected in 

 the following manner. After a Cercaria has been for some time 

 in the water, first creeping and then swimming about with mani- 

 fest restlessness, it gathers itself up into a ball, and emits from 

 its whole surface a mucous secretion which soon hardens, and since 

 inside of this mucous mass the worm, coiled up into a little ball, 

 turns round without stopping, invests it as it were in an egg-shell. 

 During this process of encysting the Cercaria invariably casts off 

 its tail, so that the capsule eventually encloses the body merely, 

 (fig. 13). For a long time I vainly wondered what could be 

 the object of this process, and, never understood what its 

 signification in Cercarian life was, until, in dissecting some 



insects, I met with a fact which 



Fig. 13. Fig. 14. suggested how I might gain the 



knowledge I sought for. In the 

 larvae of a great number of various 

 kinds of aquatic insects, of Libel- 

 lulidae, Ephemeridce, Perlidce, Phry- 

 ganidce, I found encysted Cercarice, 

 which I again discovered in the 

 same animals, after they had left 

 the water, and had been trans- 

 formed into winged insects. Not one of these encysted Cer- 

 caria lodged in an insect, was either full-grown or possessed 

 sexual organs. I only observed one other slight step towards 

 their further development ; the sexual apparatus, viz., the testis, 

 the germarium, and the copulatory organs, were already faintly 

 indicated. As, however, perfectly full-grown and sexually 

 developed Trematoda are never met with in insects, I decided, 

 after the discovery of the encysted Cercaria in them, that they 

 merely sought out the insects as a temporary resting place. 



Fig. 13. Encysted Cercaria ephemera, a. Sucker, c, d. Urinary organ. Fig. 14. 

 Abdominal extremity of a Cercaria ephemera, in which, by the casting of the tail the 

 urinary organ has been opened externally, a. Inferior expanded end of the urinary 

 organ, g. Aperture out of which the granular urine is excreted. Before I pointed 

 out the true import of this urinary organ these granules were regarded as eggs, and 

 when the urine was excreted they were thought to be laid. 



