Life-historij of some Marine Diatoms. By J. D, Siddall. 379 



chiefly due to the presence in the diatom of a large number of 

 roundish ovoid chromoplasts grouped chiefly round the interior 

 margins, but also variously disposed and free to move in or by the 

 protoplasmic cell-content. 



The clearest demonstration of locomotion by utilization of the 

 pseudopodia I have yet seen was in the case of a good average- 

 sized specimen in a trough — light, vivid black ground ; Abbe con- 

 denser ; power, 1-in. and D eye-pieces ; binocular. The position 

 and activity of the specimen were alike most favourable. In com- 

 pany with an experienced microscopist, I watched this specimen for 

 several consecutive liours. It walked deliberately, as if on many 

 stilts — which we could plainly see ; occasionally it turned right 

 over, now on edge, again flat-topped (the most frequent position). A 

 long filament, stretched in rear, would briefly check progress until 

 it stretched, broke, and then collapsed. Then releasing hold of the 

 trough, the filaments would spread out laterally, and it would float 

 freely, suspended in the water, as most probably would be the case 

 when they are netted from tlie sea. The whole distance traversed 

 by this particular specimen during the time I watched it would be 

 at least six or eight inches. Finally it sailed, as I have seen many 

 do, to the surface of the water in the trough, and I could see or trace 

 it no longer. 



No doubt now remains in my mind as to this being a veritable 

 diatom, nor as to the origin and use of the pseudopodia. They 

 are emitted by the diatom, and are not parasitic or symbiotic. 

 Apparently they are produced by the very youngest examples, 

 and later become stiffened and probably somewhat silicified, but 

 they do not even then cease to function as tactile organs, as in 

 common with the whole diatom they are coated to their tips with 

 a layer of living protoplasm. In the fully mature frustules they 

 disappear altogether. 



Specimens mounted by Mr. H. J. Waddington, Bournemouth, 

 in sea- water, formalin, and a trace of osmic acid — and by myself 

 in xylol-balsam, after some hours in nitric acid — show clearly the 

 canals in the peripheral borders through which the pseudopodia 

 are protruded. The former, as shown in pi. IV. fig. 4, also shows the 

 retracted bases of these in the canals as extensions of the cell- 

 plasm. I have also succeeded in mounting and photographing 

 specimens in the formalin medium, showing the fully-extended 

 pseudopodia and also the structm-e of the frustule. 



In general appearance and structure the diatom most closely 

 resembles Coscinodiscus radiatus, in the life-history of which this 

 may be a phase ; but, assuming that this is the first instance in 

 which the phenomenon of pseudopodial locomotion has been de- 

 monstrated in a diatom, I venture for the sake of convenience to 

 suggest for it the specific or varietal name heliozoides. 



I have seen other diatoms — notably, a little SurireUa very 



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