DURATION OF DEVELOPMENT AND OF LIFE. 591 



According to v. Siebold's investigations, the metamorphosis of the 

 Echinococcus - heads into these tape -worms takes place with great 

 rapidity. Fifteen or twenty days after feeding he found on most of 

 the heads a two-jointed body, while others (the majority, according to 

 one of my experiments) were still without segmentation. 

 Thereafter three joints appeared, and some days later (on *-* 

 the twenty-seventh after feeding) some hard -shelled eggs ii 

 with embryos were found in the last joint. Van Beneden 

 also saw the J2chinococcus-\\esids becoming ripe Tcenice 

 within four weeks after feeding, while Kuchenmeister, Echinococcus 

 who experimented at the same time as v. Siebold, dis- k ef re . th e 



r beginning of 



covered no ripe forms for eight to nine weeks. In my ex- the segmen- 

 periments I did not find mature worms till the seventh tatlon ( x 15 )* 

 week. Zenker, even eleven weeks after feeding, found them perfectly 

 developed as regards size, but still without eggs. 1 In the feeding 

 experiments of Naunyn, Pagenstecher, and Nettleship, seven weeks 

 were also required for the attainment of maturity. 



We do not know yet how long the tape- worm lives, but the insuffi- 

 ciently supported supposition of v. Siebold, that they did not survive 

 two months, can hardly be correct ; for not only does the form and 

 thickness of the roots of the hooks suggest in certain cases a greater 

 age, but we cannot suppose, against all analogy, that the productivity 

 of the worm is exhausted after the one or two proglottides. Further, 

 we must remember that the Tcenia echinococcus is, like the other cystic 

 tape-worms of the dog, found much more frequently in older animals 

 than in those still in the first year of their life. 



Tcenia echinococcus was repeatedly observed before the researches 

 of the above-mentioned investigators. It was noticed apparently by 

 Eudolphi, who found it in immense numbers in a pug-dog. Its 

 attachment between the intestinal villi and its small size led, how- 

 ever, to the supposition that it was the young of the common tape- 

 worm of the dog (T. cucumerina), which had just arisen by generatio 

 sequivoca. 2 The villi were supposed to pass directly into tape- worms. 

 Later observers also considered these worms as young forms of other 

 species. Thus Eoll claimed them as young stages of T. serrata, and 

 Diesing was inclined to refer the specimens collected by Natterer 

 from a puma (Felis concolor) from Brazil to the large-hooked tape- 

 worms of the cat (T. crassicollis), and this, too, in spite of the expressly 

 noted ripeness of the last joint. Van Beneden in 1850 was the first 

 to recognise an independent species in this worm, which he described 



1 Sitzungsb. d. med.-phys. Gesettsch. Erlangen, p. 88, 1872. 



2 "Entozoor. hist, natur.," t. i., p. 411, 1808. 



