380 Transactions of the Society. 



abundant every summer on the muddy banks of our tidal river Dee 

 — move about by the aid of short blunt protoplasmic extensions, 

 but have never, until this present instance, been able to convince 

 myself of the presence in any species of fine long pseudopodia. I 

 have noted three methods of reproduction in the Coscinodiscus under 

 consideration, viz. (1) the ordinary method ; (2) liberation from the 

 opened frustule of chromoplasts enveloped in protoplasm : these 

 gradually assume the parent form ; (3) detachment of portions of 

 the granular protoplasm which coats the outer surface of many 

 specimens. In (2) and (3) methods pseudopodia are produced on 

 the tiniest examples, before any traceable silicification of the cell- 

 wall has taken place. Many of these young forms may be found 

 in series of two to eight, or even more, like tiny wheels on an axle ; 

 ultimately these become detached from each other. 



Melosira, which is frequent in recent nettings, also emits lateral 

 protoplasmic filaments occasionally, rather shorter and blunter than 

 those of Coscinodiscus, and each with a quite translucent portion 

 midway in the length. I have mounted a specimen in formalin, 

 showing this clearly. 



Bacillaria iiaradoxa, very varied in size and number of frustules 

 in a colony, is both abundant and active, so I have had plenty of 

 opportunity to watch and wonder over its erratic and extremely 

 forcible movements. It obviously exudes living protoplasm 

 through pores on its edges, and so produces its gliding movements. 

 It is most curious to watch an isolated frustule, itself lying station- 

 ary on the slip but carrying to and fro from end to end of its sur- 

 face a particle of debris. The involute rolling-in of the colonies is 

 also apparently due to the contractile power of the upper inner 

 surface-layer of protoplasm. 



Biddidphia avrito, and several smaller species of the genus, is 

 very common in all stages of development. Eupodiscus argus, 

 Actinocyclus, several Fleurosigmas, RJiizosohnia , Nitschia, Cliseto- 

 ceros, Eucampia, Triceratium, and many other most interesting 

 diatoms, also occur living in these most interesting and prolific 

 nettings. Like most free diatoms, all of these seem to possess 

 power of movement sufficient to enable them to creep clear of sur- 

 rounding debris. The change of position is easily noticeable in 

 troughs. Some of this movement may be due to osmotic inter- 

 change between the cytoplasm and surrounding media, aided by 

 the lifting and floating power of the gaseous products of the 

 various nutritive processes ; but I am disposed to think that the 

 extrusion of protoplasm through minute pores or thin places in 

 the envelope is far more frequent as the cause. I have seen un- 

 dloubted examples of this in Biddidphia and Triceraiivm, usually as 

 stretched portions of an investing layer of protoplasm, so tran- 

 sparent and thin as to defy anything like full observation, an occa- 

 sional glimpse being possible only. Mr. Waddington's method of 



