102 ENTOZOA. 



bold regarded it as the remains of the yolk. Leuckart, in his expe- 

 riments, introduced a number of eggs into a vessel of water contain- 

 ing several small crustaceans {Gammarus Pulex). These animals 

 readily swallowed the ova, and in a few days the embryos were 

 found emerging from their shells, boring their way through the 

 intestinal walls, passing into the general cavity of the body, and 

 even into the appendages themselves. During the next fourteen 

 days the embryos within the Gammari exhibited an increase of size 

 wliich became very marked, and at the same time the central granu- 

 lar mass assumed the aspect of cellularity, its spherical outline be- 

 coming more clearly defined. In the course of the third week a 

 further metamorphosis occurs. The nuclear mass now rapidly 

 increases in bulk, and by a process of differentiation and grouping 

 of its cell-contents acquires the aspect of a compound organism. It 

 also elongates and eventually assumes the easily recognizable cha- 

 racters of a young Echinorhynchus. Thus, in Leuckart' s own 

 words, " the ultimate animal arises in the interior of the primordial 

 body, by a process which presents so close an analogy with the pro- 

 duction of an embryo, and, consequently, with the act of genera- 

 tion, that one feels inclined at once to identify it with such an act ; 

 and, therefore also, to regard the Echinorhynchus as exhibiting an 

 alternation of generation in its mode of development rather than 

 a metamorphosis." The young Echinorhynchus subsequently con- 

 tinues to increase more rapidly in size ; its' several internal organs, 

 proboscideal sac, and muscular apparatus, gradually coming into 

 view as growth advances. Eventually the young entozoon com- 

 pletely fills the Ulterior of the embryo, the latter having scarcely 

 undergone any change ; still remaining, of course, within its crus- 

 tacean host. What may be regarded as even more extraordinary 

 is the circumstance that the embryonic body next becomes firmly 

 adherent to the young Echinorhynchus, thus ultimately forming, 

 according to Leuckart, the true integumentary envelopes of 

 the adult Echinorhynchus. The original skin of the embryo, 

 however, is cast off" " as soon as the Echinorhynchus occupies 



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