MIGRATION TO THE DEFINITIVE HOST. 73 



means of the tail, approached the insect larvae, and crept all over 

 them in a restless fashion, evidently seeking something. I also 

 noticed that every now and then they remained motionless, and 

 pressed their frontal armature against the body of the larva. In no 

 case, however, were these boring operations continued, until the Cer- 

 caria happened to have lighted upon a soft portion of the integument 

 between the segments of the insect ; then they used their spine with- 

 out ceasing until they had made an aperture in the 

 skin, through which the flexible fore-part of the body 

 could be introduced. This enlarged the opening, and 

 rendered it possible for the whole body, much attenu- 

 ated during the process, to pass through the outer FlG 5^lT^n en _ 

 skin into the perivisceral cavity. The tail of the cysted Cercaria, 

 Cercaria always remained outside, and was no doubt Wlt out tai ' 

 detached by the sides of the aperture closing together after the body 

 of the parasite had passed through. Having selected for these experi- 

 ments young and delicate larvaa, I could still observe the Cercarige 

 inside their body. They invariably remained quiescent, and assumed 

 a spherical shape (Fig. 51), surrounding themselves with a cyst. The 

 frontal spine was detached during this process of encysting, and was 

 generally visible, lying close to the Cercaria, and within the cyst. 

 This spine, therefore, like the tail, is cast off as soon as its purpose is 

 fulfilled." 



The duration of the free life varies with the species. In our 

 common Cercarise it is generally short, and many species (Distomum 

 hepaticum) do not wait to make their way into the body of some host, 

 but become encysted upon water-plants and other objects. The marine 

 forms, on the other hand, remain longer in the free stage ; some, after 

 entering the bodies of worm-larvae, Copepoda, &c., devour the tissues 

 of their host, and become encysted in its empty shell (Moebius). 



In the quiescent stage the Cercarise are just like other Entozoa in 

 the second developmental period. They await transference to a new 

 host, where, if circumstances favour it, they attain maturity. The 

 changes undergone in the intermediate host in which they some- 

 times remain for years are no more than preparations for the final 

 stage, and consist mainly in a slight increase in size, and the gradual 

 formation of the generative organs. 1 



The change to the last developmental stage is then (even in species 



1 If this intermediate condition be prolonged to an unusual extent, the encysted 

 Distomum often arrives at sexual maturity, as I have noticed myeelf in Ephemerid larvae. 

 Similar cases have been observed by other naturalists, e.g., Linstow and Villot. The last 

 mentioned has published a special paper on this circumstance ( ' ' Observ. de Distomes 

 adultes chez les Insectes," Bullet. Soc. Statistque de VIsZre, t. ii., p. 9, 1868), which, 

 however. I have not seen. 



