MARINE ALGJ5. 



269 



the motion is less lively than that of the Bacterice, which 

 Dujardin and Elirenberg placed among infusional animal- 

 cules. 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. As to 

 the cause of these motions, or the mechanism by which 

 they are 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 Algye present a general appear- 

 ance which might at first sight be mistaken for plants 

 very much higher in the scale of organization. In the 

 Ulvacese, 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 

 Bryopddeoe, 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 Derhesia, 

 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 certtcin 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. — Sphacelaria 

 cirrhosa, with spores 

 home at the sides of tue 

 IrancJiUts. 



