262 THE MICROSCOPE. 



The existence of delicate threads passing from the 

 central mass to the enveloping cells, and the appearance 

 occasionally of little particles having molecular motion, 

 serve to show that the contents of the enveloping cell are 

 less of a gelatinous consistence, than of a fluid nature. 

 And the continuity of the primordial cell-wall with the 

 filaments proves it is surrounded only with a layer of 

 protoplasm, and is not inclosed in a dense membrane 

 of cellulose. The most distinctive characteristic of the 

 primordial cell, and what appears to constitute its most 

 essential importance in the life of the cell in general, but 

 particularly in that of the zoospore, consists in its being 

 the contractile element of the vegetable organism — that 

 is to say, that from an intrinsic activity it possesses the 

 faculty of altering its figure, without any corresponding 

 change in volume. 



The Protococcus pluvialis has true motile organs, 

 namely, two long vibratile cilia arising from the primordial 

 cell (fig. 144, B, a\ which, passing through two openings in 

 the enveloping cell, move about in the water. These organs, 

 during the life of the cell, move so rapidly, that it is 

 then difficult to perceive them ; they are only recognisable 

 by the currents they produce in the water. But when 

 the motion is slackened they are evident enough. They 

 are also rendered very distinct by iodine. They are always 

 placed upon the extreme point of the conical elongation, 

 on the anterior end of the primordial cell, and in such a 

 manner as to appear to be immediate continuations of its 

 substance ; and as that process itself consists of protoplasm, 

 it is evident that the cilia must be regarded also as com- 

 posed of the same substance. They resemble, in some 

 respects, the so-called proboscis of certain Infusoria, such 

 as Euglena and Monads, and do not differ very materially 

 from the non- vibratile, retractile filaments of Acineta and 

 Actinophrys. 



It is only that portion of the vibratile filaments beyond 

 the enveloping cell that exhibits any motion, the portion 

 within the outer cell being always motionless, and in that 

 part of their course the filaments appear to be surrounded 

 with a sheath. This seems to be the case, not only from 

 the greater thickness at that part, but also from the cir- 



