cxxxvi Introduction. 



cylindriform, and possessed of the power of oscillatorial move- 

 ment. 



The impregnated ova of the ChaetognatJia are developed without 

 going- through any metamorphosis, and without the formation of 

 any ciliated coat. The first stage of the embryonic life of the 

 Acanthocephali is constituted by a form armed anteriorly with 

 deciduous spiny hooks, and containing internally a mass of germ- 

 inal matter^ out of which the internal organs are developed after 

 the animal has found its way into the appropriate portion of the body 

 of its first host, and discarded its provisional organs. The sexual 

 condition is only attained in the body of a second host, which is a 

 Vertebrate, whilst the first is ordinarily (or always?) an Inverte- 

 brate animal. In the free Nematoids the development appears 

 to be direct ; but in the parasitic species it is complicated by the 

 fact that the embrj^o is not developed in the same medium as that 

 in which its mother lived, but migrates either into some other organ 

 of the same animal, or into the exterior water or damp earth, 

 whence it may find its way back into the body of the animal 

 whence it was extruded, either directly or after a migration into 

 some second host. A parasitic Nematoid may be undistinguishable 

 in its free stage from a true free Nematoid, being of the same 

 shape, and feeding, growing and moulting in the same way. Ascaris 

 nigrovenom, when set free from the body of the frog where it is 

 produced parthenogenetically, attains to sexual perfection in the 

 free state which they spend in moist earth. The development of 

 the Guinea-worm, Filaria Medmensis, within the subcutaneous 

 tissues of the human subject, would appear to be similarly partheno- 

 genetic, and probably alternative with a free stage, which, though 

 it has not been observed, may be supposed to be similar to that of 

 the Ascaris nigrorenosa. Parthenogenesis has also been observed 

 in a Nematoid worm infesting Limax ciuereus ; and Sphaendaria 

 bomhi, if Schneidei-'s explanation of Sir John Lubbock^s account 

 of this animal be correct, must be considered as furnishing a fourth 

 instance of asexual reproduction in this class. No instances of 

 reproduction by gemmation or metagenesis have been observed in 

 the adult Nematelminthes ; the power of repairing injuries has, in 

 the free Nematoids, been found to be low, or absolutely none ; 

 and the breaking of the body of the Guinea-worm, Filaria MecU- 

 nensis, appears to entail its death even under the condition of its 



