THALLOPHYTA. 635 



same colonies, but a little later than the androgonidia, about thirty much larger 

 spherical cells, the oospheres, are developed, and these are duly fertilized, but by 

 spermatozoids derived from another colony. The zygote has a sculptured exine. 

 Parthenogonidia are not found in those colonies which produce gametes. 



Volvox aureus ( = F. minor), the commoner form, is usually much smaller than 

 V. Globator, and has rounded cells more widely separated and connected by very 

 delicate processes. But perhajis its most striking characteristic is the very great 

 variability in the number and distribution of the reproductive cells. The partheno- 

 gonidia, which vary in number from one to sixteen, may either occur alone or in 

 one colony with androgonidia or oospheres, or both. Most of the sexual colonies 

 are dicecious, though this is not always the case. The colonies containing androgo- 

 nidia unaccompanied by other reproductive cells often develop very numerous (up 

 to 1100) spermatozoid bundles, the androgonidia forming one-third of all the cells of 

 the colony. The spermatozoids differ from those of V. Globator by their larger size, 

 by their terminal flagella at the end of a shorter beak, and by the possession of a 

 well-developed leaf -green chromatophore. We must, therefore, consider V. aureus 

 as not so highly developed, in some respects at least, as V. Globator. 



A Volv ox-colony always swims in the direction of a given axis passing through 

 its body, and at the same time rotates to the right or left about an axis which is 

 inclined obliquely to the antero-posterior axis. The eye-spots of the vegetative cells 

 are much better developed in the anterior half of the colony, and are always 

 situated on the side of the cell nearest the anterior pole. These facts tend to 

 support the view of the function of eye-spots in general suggested above. 



Volvox stands at the head of the series of colonial (ccenobe-forming) organisms 

 which we have been tracing, a series diverging from a Chlamydomonas- or SphcB- 

 rella-\\kQ type, and whose successive forms gradually increase in size, complexity, 

 and sexual differentiation. Volvox itself has been well spoken of as " the culmina- 

 tion of Nature's attempt to evolve a higher organism out of a coenobe ". It was an 

 attempt which failed, or rather which could not be carried any further than Volvox 

 itself. A delicate, easily-ruptured Foit>ox-sphere could certainly not continue to 

 exist if it were much more than a millimetre in diameter. As it is, the wall is often 

 split, and all sorts of smaller organisms get inside, resulting in the more or less 

 speedy collapse of the Volvox-Qoloiry. 



But there are other series diverging from the Chlamydomonade^, and some at 

 least of them have followed lines on which it was possible for higher and more 

 varied plant-forms to be developed. 



At the first stage along one of these lines of descent we find ourselves among 

 forms in which the dominant phase of the life-history falls in a resting stage, either 

 fixed or freely floating in the water. From this resting stage motile forms (zoo- 

 spores), corresponding with the free-swimming Chlamydomonas individuals, are 

 directly developed. These zoospores, after a short period of swarming, come to rest, 

 often fixing themselves by their anterior end to some solid object. With little or 

 no change in the constitution and appearance of the cell the main portion of the 



