the Origin of Intestinal irorms, 347 



part of its life in the abdominal cavity of fishes, and in this 

 state has neither reproductive organs nor head ; but that if 

 swallowed by sea-birds, and perhaps by seals, it obtains these 

 essential parts, in the intestines of these animals. The changes 

 of structure, however, cannot interfere with the fact, that the 

 Entozoa sometimes undergo a change of abode from one ani- 

 mal to another, but may be adduced as proofs that such changes 

 of abode are sometimes necessary for the perfect development 

 of the intestinal worms. Even Rudolplii, the zealous defender 

 of the spontaneous origin of the Entozoa, was so impressed by 

 this fact, that he advanced this view, which he had considered 

 so erroneous when proposed by Bremser. {Vide Entozoorum 

 Synopsis, p. 596.) 



Sect. 3. The Entozoa or Intestinal JVorms are very commonly^ 

 subject to Metamorphoses. That metamorphosis occurs could 

 scarcely fail to be deduced from the general observation, that, 

 wherever young ones were found in the parent intestinal worm, 

 they never resembled that parent, and sometimes entirely dif- 

 fered from it. 



As one instance of this, we may mention the observation of 

 Dr Jacobson at Copenhagen on Filaria medinensis^ communi- 

 cated by Mr Blainville in the " Annales des Sciences Natu- 

 relles." In the interior of this worm he fovmd a great number 

 of young living worms, as Rudolphi had found before (Synopsis, 

 p. 206), but they were so dissimilar to their parent, that he 

 doubted Avhether they might not be parasitical worms within 

 the parasite. Again, in the Echinorhynchi^ which also pro- 

 duce their young alive, precisely the same remark of the dif- 

 ference between the young and the parent has been made by 

 Dr Siebold (Burdach Phys. 2 edit. 1. c). The same observa- 

 tion may be applied to the Cestoidea, in many of which the 

 young have been observed in the eggs (Siebold, 1. c.) ; the 

 head in all being first formed and provided with six hooks. Of 

 the embryo of Taenia solium a delineation has lately been 

 given by Mr Dujardin in the Annal. des Sc. Nat. 1838. 



The most astonishing instance of metamorphosis, however, 

 if I mistake not, is that observed by Dr Siebold, and related in 

 Wiegmann's Archiv (vol. i. 1835). It refers to the Monostoma 

 nmtabUcy which frequents various parts of certain water-fowls, 

 produces its young alive, these not having the slightest re- 



