MARINE ALGJE. 



269 



membrane. Their motion is not less lively than that of 

 the Bacterice, which Dujardin and Ehrenberg placed among 

 infusional animalcules. To observe the movements of the 

 filaments, the very uppermost surface ought to be brought 

 into focus, leaving the margins rather undefined, bearing in 

 mind that the filament is not a flat but a cylindrical body. 

 Certainly, with regard to its movement, or the mechanism 

 by which it is effected, nothing positive is known. 



The Bacillaria paradoxa is by far the most interesting 

 specimen of the genus ; the movements of which are very 

 remarkable, and so little understood that it is rightly 

 called paradoxical. 



The Marine Confervoid Algje present a general appear- 

 Aiice which might at first sight be mistaken for plants 

 very much higher in the scale of organization. In the 

 Ulvaceas, the frond has no longer the form of a filament, 

 but assumes that of a membranous expansion of the cell. 

 These cells, in which zoospores are found, have an in. 

 creased quantity of green protoplasm 

 accumulated towards one point of the 

 cell-wall; and the zoospores are ob- 

 served to converge with their apices 

 towards the same point. In. some 

 genera, which seem to be closely re- 

 lated in form and structure to the 

 Bryopsidece, we notice this important 

 difference, that the zoospores are de- 

 veloped in an organ specially destined 

 to this purpose, which presents pecu- 

 liarities of form, distinguishing it from 

 every other part of the branching 

 tubular frond. In the genus Derbesia, 

 distinct spore cases are seen, a young 

 branch of which, when destined to be- 

 come a sporecase, instead of elongating 

 indefinitely, begins, after having arrived 

 at a certain length, to swell out into 

 an ovoid vesicle, in the cavity of which a rapid accumu- 

 lation of protoplasma takes place. This is then separated 

 from the rest of the plant, and becomes an opaque mass, 

 surrounded by a distinct membrane. After a time a 



Fig. 147. Sphacelarta 

 cirrhosa, with spores 

 borne at the sides of tlie 

 branchlets. 



