384 THE DEVELOPMENT OF CESTODES. 



differences obtain in regard to their number. We know tape-worms 

 with only three or four joints, and some with as many thousands, hence 

 the period of a joint's development must also vary greatly. In the 

 larger tape- worms, several months pass before the first joint is libe- 

 rated, while in other cases only a few days are required. 



At first sight it looks as though the head remained almost un- 

 changed, and was only of importance in the formation of the chain in 

 so far as it furnished, as it were, the raw material for the latter ; but 

 this view is not confirmed by closer observation and comparison, 1 

 for not only does it grow for a while in its Tsenioid state, but its 

 hooks often become larger and thicker by the deposition of new 

 chitinous matter on the roots and on the base. The changes are 

 indeed usually but slightly marked, so that they escape observation 

 in the majority of cases. Tcenia echinococcus is, however, an excep- 

 tion, since the differences in the size and shape of the hooks at 

 different stages are so striking that for a while they caused perplexity 

 in the study of the nature and life-history of the form in question. 

 Among the Bothriadse also we know of a form (Echineibothrium) 

 which does not complete the development of its armature till it 

 reaches the tape-worm stage. 



In all cases the metamorphosis of the bladder-worm into the tape- 

 worm takes place only after the transference of the former into the 

 intestine of a suitable host. In its original abode the bladder- worm 

 remains what it was. It grows to a certain size, and may live on 

 perhaps for years, but a further development is impossible in this 

 environment. 



As to the age which bladder- worms as such may attain, we have as 

 yet but few results, and what we have relate only to the forms found 

 in vertebrates. The Echinococcus is credited with the longest life. 

 We know of individuals who have suffered from this parasite for 

 thirty years and more. A deduction from this as to the other bladder- 

 worms is, however, illegitimate, for in Echinococcus we have to do, not 

 with single individuals, as in the Cysticerci, but with generations, of 

 which the oldest have long since perished, while the young forms are 



1 This would be still more evident, if it be true, as Megnin has just asserted (Comptes 

 rendus, t. xc., p. 715, 1880), that after a longer or shorter existence the head loses 

 its hooks and suckers, and is finally so thoroughly absorbed that only the chain of 

 joints remains. In this acephalous state the latter persists till the youngest joint is ripe, 

 and the life of the tape-worm thus comes to an end. This has indeed been observed by 

 Me'gnin only in a few species (Tcenia infundibuliformis of the fowl and Tcenia lancedata 

 of the goose), but he does not hesitate to infer that the same may be true of the other 

 species, though the length of life is often very long. [In this connection we may not un- 

 fitly recall the curious Idiogenes otidi* of Krabbe ( Vid. Meddel. nat. Foren. Kjobenhavn, 

 p. 122, 1867), one of the Tseniadae, which is known to be headless and possessed of a very 

 peculiar anterior segment R. L.] 



