TRANSITION TO THE ADULT STATE. 379 



merely because it attains in the Cysticercoid condition that maturity 

 which is in other cases only reached at a later stage of development 

 wanting in Archigetes. In order to attain this definitive developmental 

 stage the tape-worms must be transferred from their former host to 

 some other suitable animal. Along with their hosts, or with the 

 portions which they inhabit, they are imported into the intestine of a 

 new host, where, after greater or less alterations, they become adult 

 tape- worms. "With the exception of Archigetes, the sexually mature 

 Cestodes occur only in the vertebrates. 



The transition into the sexually mature worm occurs most simply 

 in the Ligulidse, which, as larvae, are already comparatively mature, and 

 have their sexual organs even then almost completely developed. 

 Twenty-four hours after they have passed into the intestine of a duck 

 or some water-bird, one finds them with eggs fully developed, pro- 

 vided of course that they previously possessed the requisite differen- 

 tiation and size (at least 10 cm.). 1 In other cases the worms are 

 digested, or are expelled unchanged with the faeces. Even in their 

 sexually mature state the Ligulidae remain but a few days in the 

 alimentary canal of the bird ; after the course of a week or even earlier 

 they succumb to the same fate as the larval forms, being either 

 expelled or altered by digestion, which first affects the posterior end. 

 Donnadieu estimates the average duration of the parasitism of 

 Ligulidse only at two and a half days. In water of ordinary tempera- 

 ture these animals remain living for eight or ten days. As regards 

 their external appearance, the only change on the acquisition of 

 sexuality is the considerable elongation and narrowing of the body. 



In the case of the isolated Tetrarhynchus-heads, the progress of 

 events is somewhat more complicated. After they have been trans- 

 ferred, along with their intermediate host, into the alimentary 

 canal of a shark or ray, the passage into the final stage is accom- 



to man from fish, especially the pike. R. L.] I may also remark that Megnin and Moniez 

 have lately maintained for the Tcenice a continuous development without change of host 

 and without cystic state a supposition based essentially on the fact that the number of 

 known bladder-worm forms is much too small in proportion to the great number of the 

 existing species of tape-worms. They forget that the tape-worm hosts often devour 

 hundreds of animals before they find a bladder-worm host ; in other words, that the 

 bladder-worms are very sparsely distributed, especially in the lower animals, and therefore 

 often escape our investigation. At any rate this assertion is not worthy of much con- 

 sideration until it is established by direct observations. 



1 See Donnadieu, loc. cit., and Duchamp, Ann. sci. not., t. vii., No. 7, 1876. The 

 latter observed the attainment of sexual maturity even in some worms transferred experi- 

 mentally into the body-cavity of a dog (Comptes Rendus, t. Ixxxvi., p. 493, 1878). It can 

 therefore hardly be doubted that it is primarily and principally warmth which brings them 

 to ripeness ; nor is it necessary to have the whole worm even separate pieces become ripe 

 in the intestine of the bird. 



