AHIIFICIAL CULTUKE OF MAUINE PLANKTON ORGANISMS. 413 



and wei-e also easily seen on the megafrustules (cf. below), 

 which developed in it, but in none of the several generations 

 of cultures started from this one have we been able to find 

 any traces of marking- at all. The genus seems to be' in con- 

 sidenhle confusion, and it is probable that the conflicting 

 descriptions given by different observers are due to variations 

 in what is really one species. 



Persistent cultures of Coscinodiscus excentricus Ehr. 

 have also been obtained, and it is interesting to note that this 

 diatom sometimes forms chains, but they are i-ather excep- 

 tional. These chains are never as long as those commonly 

 found with Thalassios ira, two or four cells only being the 

 rule. The filament joining the valves is also finer and more 

 easily bi-uken. The two species are quite distinct, and culture.s 

 of them can be discriminated by a practised eye. 



Two species of the genus Biddulphia are commonly 

 met with in our cultures, namely Biddulphia mobiliensis 

 (Bail.) Grun. and Biddulphia regia M. Schultze. These 

 two forms are generally regarded as one species, but Osten- 

 feld (54) has recently shown that they are really distinct. 

 We have obtained persistent cidtures of both forms from 

 several different samples of plankton, and the two species are 

 easily recognisable, never merging into one another. When 

 Petri dishes, inoculated from plankton (see p. 367), contain 

 both species, the colonies can be easily distinguished with a 

 small hand lens. 



The most generally accepted theory of the reproduction of 

 the Diatomaceae is briefly that the cells divide by simple fission, 

 but on account of the rigid character of the cell-walls each 

 division necessitates a deci-ease in size of the new valve, since 

 this must always be formed inside the old valve. So the 

 frustules gradually get smaller and smaller as multiplication 

 proceeds, thus necessitating some process by which the 

 original size can be re-established. This takes place by the 

 formation of what are known as auxospores, which ultimately 

 form megafrustules, and these in turn multiply b}^ division 

 until the minimum limit of size has again been reached. 



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