220 Dr. J. D. Macdonald on the Distribution of [Mar. 20, 



earliest genetic type of beings. With the exception of a nucleus and a 

 contractile vesicle, Amoeba itself may have sprung from Protamoeba ; and 

 the finally encysted jelly-globules of Protomyxa and Myxastrum breaking 

 up into naked amoeboids, or pseudonavicellaB liberating them, very 

 strikingly suggest the source from which the Gregarince 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 operation, 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 such 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 w T e 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 Grregarinida3. I have already noticed the 

 affinity of the Grregarinians themselves to Protomyoca and Myxastrum 

 amongst the Monera ; but when we find the hooklets of Taenia and the 

 sucker-pits of Tamia and Bqthryocephalus shadowed forth in Hoplo- 

 rhynchus and Actinocephalus respectively, we can scarcely help acknow- 

 ledging the alliance here indicated. In the Gregarinidge, moreover, there 

 is not only a distinct external integument, but Yan 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. 



jSTow, although the Thalassicollidse are not parasites, the genus Thalassi- 

 colla and the Gregarince alone of all the simple Protozoa take up their nutri- 

 ment in solution, after the manner of the compound forms, namely the 

 Porifera, restricted Polycystina, and Poraminifera. This fact, I think, is 

 significant, as suggesting the derivation of Gregarina from some such 

 original as Thalassicolla, as it does not seem natural to suppose that the 

 former, which is so essentially an Entozoon, could have been descended 

 from a stock capable of assuming solid food in the outer world. 



Dr. Carpenter unconsciously gives us the weight of his opinion in the 

 following quotations from his valuable work on the microscope. On 

 page 449 he says, speaking of Sjphcerozoum, "Towards the inner surface of 

 this (the outer) coat are scattered a great number of oval bodies resembling 



