470 PARASITES OP ANIMALS 



embryo of this parasite is broad and obliquely truncated at the 

 ventral surface anteriorly, being gradually narrowed to a blunt 

 point posteriorly, and at the front part, on each side of the 

 middle line, there are five or six spines biserially disposed. 

 Similar characters are seen in E. filicollis. Prof. Leuckart 

 introduced a number of eggs into a vessel of water con- 

 taining several small crustaceans (Gammarus Pulex). These 

 little 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, then 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 ; and in course of the third week 

 a further metamorphosis caused the embryos to assume the 

 readily recognisable characters 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 

 BO close an analogy with the production of an embryo, and, 

 consequently, with the act of generation, 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 metamor- 

 phosis/' 



The young Echinorhynchus afterwards grows rapidly, its 

 several internal organs, proboscideal sac, and muscular apparatus, 

 gradually coming into view. At last the young entozoon com- 

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

 undergone any change, and still remaining, of course, within its 

 crustacean host. What may be regarded as even more extra- 

 ordinary is the circumstance that the embryonic body next 

 becomes firmly adherent to the young Echinorhynchus, thus 

 ultimately forming the true integument of the adult Echinorhyn- 

 chus. The original skin of the embryo, however, is cast off 

 " as soon as the Echinorhynchus occupies the whole interior of 

 the embryo." After this the sexual differences become clearly 

 established. Leuckart remarks that the passage of the young 

 Echinorhynchi into their ultimate host is probably unattended 

 by any striking changes, whilst the metamorphosis of the embryo, 

 as thus far detailed, occupies a period of about six weeks. In 

 general the crustacean hosts appear to suffer little from the 

 borings of the embryo parasites, but when the latter have 

 assumed the Echinorhynchus - condition and happen to be 



