378 Transactions of the. Society. 



clear of tlie obscuring debris, and then examined with 1-in. and 

 blapk-ground illumination, permitted observation under favourable 

 conditions. Bacillaria was very numerous and active, but one of 

 the forms of Coscinodiscus, a small yellow one of about -^^q in. 

 in diameter, attracted my special attention by its habit of standing 

 elevated as if on stilts, and not resting on the surface of the glass. 

 It is in reference to this diatom that I venture to record the follow- 

 ing observations. 



By patient manipulation of the light I was able at last to 

 glimpse most delicate pellucid pseudopodial filaments, upon the tips 

 of which the Coscinodiscus supported itself, and by the movements 

 of which it could walk about, turn over, or float freely in the water. 



To further more minutely study this, to me, unique pheno- 

 menon of the emission of such pseudopodia by an obvious diatom, 

 I put " dips " containing specimens upon thin glass slips, illumi- 

 nated them with the Leitz oil-immersion paraboloid, and examined 

 them with powers from ^ to ^^g in. The appearance under these 

 conditions was as represented in the accompanying figures (pi. III. 

 figs. 1 and 2), only, of course, on a brilliant black ground — without 

 which, by the way, the pseudopodia are too pellucid to be discerned. 

 From both top and bottom margins of tlie diatom over forty ex- 

 ceedingly delicate long unbrauched pseudopodia were extended. 

 "With the most careful illumination and highest power I could just 

 make out their minutely granular structure, but could not detect 

 any circulation as in the pseudopodia of most Ehizopoda. The 

 length varied, sometimes extending to five or six times the diameter 

 of the frustule. They were obviously emitted from a living layer 

 of transparent slightly granular protoplasm, through the top and 

 bottom peripheral margins. The brownish-yellow colour was 



EXPLANATION OF PLATE IV. 

 Coscinodiscus heliozoides. 



Fig. 1. — Photograph. Half-inch objective and Leitz oil-immersion black-ground 



condenser ; shows pseudopodial filaments. 

 ,, 2. — Photograph. Zeiss 2-nim. apochromatic, focused to show structure of 



border and bases of filaments. 

 ,, 3. — Photograph. Zeiss 2-mm. apochromatic, mounted in balsam after 24 



hours nitric acid. Filaments entirely dissolved ; surface-markings and 



border show well with a pocket-lens. 

 ,, 4. — Drawing. Showing canals through border and bases of pseudopodia as 



extensions of cytoplasm in them. The large black spots are chromo- 



plasts. Cell contents slightly contracted and cleared by formalin (from 



a slide prepared by Mr. H. J. Waddington). x 800. 

 ,, 5. — Photograph. Zeiss 2-mm. and No. 8 compensating eye-piece ; same 



specimen as in fig. 2 ; shows same details and also surface-markings. 



X 1260. 



Figs. 1, 2, 4, 5 are from specimens mounted in sea-water and formalin, with- 

 out any attempt at cleaning. The whole outer surface, including the filaments, 

 is coated with protoplasm. 



