the Invertehrata in relation to Evolution. 393 



more simple structure than that of the perfectly homogeneous 

 and jelh^-like Monera. Indeed the evolution of any of the other 

 primitive forms from a plastic source like this is quite conceivable, 

 though of course we have no actual means of observing such a 

 transmutation. 



Moreover the development of amceboids in some part of the 

 life-history of most Protozoa would appear to stamp that form 

 as the earliest genetic type of beings. With the exception of a 

 nucleus and a contractile vesicle, Amoeha itself may have sprung 

 from Protamceha ; and the finally encysted jelly-globules of Proto- 

 myxa and Mt/.rastrum breaking up into naked amceboids, or 

 pseudonavicellcB liberating them, very strikingly suggest the source 

 from which the Greyarince may have been evolved. 



The valuable researches of Mr. Archer, of Dublin, have brought 

 to light many very interesting freshwater Protozoa, thus much 

 augmenting our materials for comparison, and adding new zest to 

 inquiry as to their natural affinities or their probable origin and 

 derivatives. 



If evolutionary forces are admitted to be in constant opera- 

 tion, it would be hard to say that any two existing forms should 

 stand to each other in the relation of source and product. It 

 would perhaps be safer to say that existing forms have taken 

 their origin from siich forms as are still in existence ; for as it 

 is but reasonable to suppose that in the lapse of time all the 

 members of the primary type must have undergone some change, 

 the persistence of that type through all in its primitive state is 

 difficult to conceive, though, for any thing we yet know, this may 

 be the case. 



Without indulging in this theme further, if we now seek for 

 the most probable derivatives of definite types of Protozoa, some 

 remarkable facts strike us, first, in relation to the Cestoid worms, 

 as bearing upon their possible derivation from the Gregarinidae. 

 I have already noticed the affinity of the Gregarinidans them- 

 selves to Protomy.va and Myxastrmn amongst the Moifura ; but 

 when we find the booklets of Ta"nia and the sucker-pits of Tmnia 

 and Bothrioc£phalus shadowed forth in Hojjhrhi/iicJms and Actino- 

 cephalus respectively, we can scarcely help acknowledging the 

 alliance here indicated. In the Gregarinidae, moreover, there is not 

 only a distinct external integument, but Van Beneden has lately 

 demonstrated the existence of circular muscular fibres on its 

 inner surface ; a similar habit of life in both cases is also very 

 significant. Nor would it be inconsistent to regard the Trematoda 

 and Nematoidea as further developments of the same series of 

 essentially internal parasites. 



Now, although the Thalassicollidse are not parasites, the genus 

 Thaldssicolla and the Gre;/an.nce alone of all the simple Protozoa 

 take up their nutriment in solution, after the manner of the com- 

 pound forms, namely the Porifera, restricted Polycystuia, and 

 Poraminifera. This fact, I think, is significant, as suggesting the 

 derivation of Gregarina from some such original as Tlmlassicclla, 



