112 GENERAL HISTOEY OF THE INFUSOEIA. 



or only an elongated neck bearing one or more cilia (JlabeUa) to serve as 

 locomotive organs. 



How greatly their figure and size are dependent on the external influence 

 of light, is well shown by some recent researches of Cohn on Stephanosphcera 

 (Nov. Act. Acad. Curios, xxvi. 1857). On placing specimens of this organism, 

 some in transparent glass vessels, others in semitransparent and green ones, 

 others in porcelain, and others again in perfectly opaque cups, the modifica- 

 tions in size and figure, according to the intensity of light they received, 

 were altogether incredible. In the opaque vessels, where they got Little light, 

 the green cells remained delicate, small, and widely dispersed, whilst in the 

 transparent glasses, under sunlight, they became many times larger and 

 crowded together, and their figure fusiform, irregular, and produced into 

 numerous protoplasmic processes. Indeed, on placing two portions of the 

 same collection of Stej^hanosj^hcerct-glohes, the one in a transparent, the other 

 in an opaque vessel, the swarming individuals in the two will be found so 

 unlike that they might be readily conceived to be different species. 



The outline is fixed where the organism has a firm envelope ; and most of 

 the Phytozoa have such in one phase of their existence, viz. when they undergo 

 the encysting-process. We are not acquainted with the entire history of 

 many genera ; but from what we know of some, we may argue by analogy of 

 all, that in the earliest stages of existence these cellular organisms have no 

 distinctly organized wall, although they may have a pellicle derived from the 

 contact of the protoplasm, of which they consist, mth siuToimding media, — a 

 mere superficial induration, but no separable membrane. Such is true of the 

 individual cells of Vohox (XX), of Euglena (XYIII. 45, 46), and of Monads 

 (XYIII. 1 to 28) in general. Subsequently a cell-wall, the primordial mem- 

 brane or sac, may be produced, distinct and separable from the contained 

 substance. Furthermore, many examples do not stop here, but proceed 

 to throw out a second wall exterior to the last-named, separated frequently 

 from it by a small interspace, and having a much denser and firmer consist- 

 ence. The cell, or, as Prof. Henfrey calls it in the case of Pandorina, the 

 gonidimn (XIX. 61), encloses itself, in fact, within a cyst (XIX. 69), and in 

 so doing mostly alters its form materially, loses its previous animal characters, 

 becomes ' still,' and at the same time quaUfied to sustain Hfe under various 

 adverse external influences, and to continue the species by an ulterior act of 

 development. In all this we trace an exact parallel with the history of the 

 spores of the lower Algae ; and there is no question that many of the Phytozoa 

 are no other than spores, sporozoids, or zoospores. Moreover, it is equally clear 

 that many Monadina and Cryptomonadina described by Ehrenberg are but 

 two phases of one and the same organism. 



Not a few Phytozoa present an additional coveiing in the shape of a muci- 

 laginous layer. This is found in isolated species, as Protococcus pluvialis, 

 and generally in aU the aggregated forms ; indeed, it is the principal agent 

 in the construction of the latter. It has generally been assumed that this 

 mucilaginous investment is an extracellular product, without a definite bound- 

 ary ; but Cohn (on Protoccocus, R. S. 1853) has a long argument to prove 

 that the true cell is represented by it, conjointly with the included, coloured, 

 apparent cell. Thus, he writes (p. 531) — '' Neither of these bodies are true, 

 perfect cells, inasmuch as the fii^st wants the primordial utricle, and the second 

 is without the true cell-membrane. The two together would represent the 

 perfect cell." Again, it is stated in the same page, " that the internal globular 

 body is not surroimded by any special cellulose-membrane, but only by one 

 readily destroyed by chemical or physical agency — probably nothing more than 

 a dense layer of protoplasm. On the other hand, the external membrane. 



