620 J. G. GEENFBLL. 



that the pseudopodia are a vegetable growth covering every- 

 thing in the field. But here the Heytesbury set are conclu- 

 sive : on a single slide you may have some hundreds of diatoms 

 of various kinds ; about 200 may be the small Melosira, nearly 

 every one of which will have pseudopodia, while nothing else 

 in the field has any. Besides, botanists will, I think, agree 

 that there is nothing plant-like in these forms. Another sug- 

 gestion was that they are like the filaments of Polysiphonia. 

 But these latter never branch and are not granular, and could 

 not form connecting bands. 



Another class of suggestions was that these pseudopodia do 

 not belong to the diatoms, but to an investing animal like 

 Vampyrella, which devours Gomphonema. But if they belong 

 to an investing animal, where is the animal which invests? 

 Vampyrella, any way, is visible both when wandering in search 

 of diatoms and when investing. Besides, Vampyrella projects 

 its pseudopodia from any part of the diatom, while here 

 they are mainly confined to a definite tract. And Vampy- 

 rella and other predatory animals do not have pseudopodia 

 so regularly symmetrical, nor so constant in number. I have 

 seen what appeared to be Leidy's Biomyxa vagans, which is 

 something like Vampyrella, investing diatoms. And here also 

 the pseudopodia had no relation whatever to the structure of 

 the diatom, while the animal itself was clearly visible outside 

 of the diatom. But, strongest proof of all, Vampyrella devours 

 the diatom and kills it in a couple of hours ; while these 

 diatoms lived for five weeks in one bottle, quite healthy all the 

 time, and dead ones were not found. 



I do not think that any theory based on Vampyrella or vam- 

 pyrelloid animals (as suggested at the British Association) will 

 explain the facts. If the pseudopodia are foreign to the 

 diatom, it would have to be a case of symbiosis between the 

 diatoms and unknown invisible animals. I know of no similar 

 case. 



All the phenomena seem to me to point to these pseudo- 

 podia being filamentous extensions of the cell protoplasm, pro- 

 bably strengthened by cuticular deposit. 



