364 THE DEVELOPMENT OF CESTODES. 



rhynchus, with which we have become more intimately acquainted 

 through Aubert, 1 and also the young form of the Tcenia cucumerina 

 of the dog. 



Two closely related species of the former live in the tench the 

 one between the intestinal villi, the other in the gall-bladder ; in 

 situations, therefore, which are only rarely inhabited by young Hel- 

 minths. They exhibit, also, a habit very unusual for a bladder- 

 worm, namely, that of occasionally stretching the head out of the 

 caudal bladder and creeping about inside their place of abode, like an 

 adult tape- worm. Even in this state, however, the adherent caudal 

 bladder, which is sharply separated from the rest of the body both 

 externally and histologically, attests undeniably their Cysticercoid 

 nature. To reach their adult state, they have to pass into the in- 

 testine of the heron, where, according to Krabbe, they are to be 

 recognised by the characteristic structure of the hooks, as Tcenia 

 macropeos and T. unilateralis. As to the position of the head in 

 the invaginated state there can be no doubt, after Aubert's description 

 and figures. 2 "The points of the hooks are," he says, "turned to- 

 wards the posterior end of the animal, while the central fixed portions 

 lie towards the unfolding of the fat-sac (i.e., towards the point of 

 invagination of the caudal bladder), and towards the same point be- 

 tween it and the circle of hooks lie the suckers." All this agrees 

 exactly both with the formation of a typical Cystieercus and with the 

 state of affairs in the Cysticercoid of Tcenia ellip- 

 tica (=T. ciwumerina), which, as is well known, 

 undergoes its development in the body-cavity 

 of the dog-louse (Trichodectes canis) (Fig. 211). 



The latter may be ranked by the side of 

 Gyporhynchus, on the ground of the morpho- 

 logical similarity between the two. Both arise 

 in similar fashion from a sac, and from a Tcenia- 

 head invaginated therein. The resemblance, 

 however, stops here ; for, in their further rela- 

 tions, the two forms differ widely. This is the 

 Fro. 211. Cysticercoid of ca se at least with the external body, which we 

 I(x60) 'have just called the "sac." In the Gypo- 

 rhynchus this exhibits a distinct caudal bladder, as in the other Cysti- 

 cercoids which we have discussed. This bladder is in form and 

 histology sharply distinguishable from the rest of the body, and re- 

 tains its independent character even in the outstretched state. In 

 the Cystieercus of Tcenia elliptica, however, the bladder seems in 



1 Zeitschr.f. wiss. Zool., Bd. viii., p. 274, 1857. 



2 Loc. cit., p. 285, Tab. x., Fig. 7. 



