THE CONTRACTILE VESICLE OF PHYTOZOA. 155 



through the shell of its primogenitor, and swims away by the 

 help of the cilia of its own zoospores. 



Thus it would appear that not only by its structural relations, 

 but also by its developmental process, Volvox stands in close 

 affinity to the snow-plant ; but there are certain other beings 

 which lend still more powerful evidence as to its plant-nature. 

 There is a so-called fresh-water plant, Gonium, which differs from 

 Volvox only in being flat instead of globular ; just as if a Volvox 

 were cut into halves, and each hemisphere flattened out into a 

 leaf-like body. The Gonium usually grows in minute square 

 patches, and has but a few zoospores ; but there are others of its 

 aquatic relatives, such as Tetrasporce, which develop to a larger 

 size, and resemble the faded leaves of curled lettuce. From these 

 the transition seems indubitable to other leaf-like Alga?, but 

 which do not possess ciliated zoospores except at stated periods. 

 Such are Monostroma, whose cells are in a single pavement- 

 like layer, or Ulva, already mentioned in another place, (p. 143,) 

 whose cells are in two layers, or Laminaria, Chorda, &c., as I 

 have already indicated (p. 143) when speaking of Thuret's inves- 

 tigations. 



All this would appear to be perfectly clear, were it not for one 

 fact which I have not yet mentioned, and that is to this effect. 

 Certain of these so-called Algas possess an organ which hitherto 

 has been looked upon as exclusively an animal property, I 

 mean a contractile vesicle. Not only does Volvox possess this 

 organ, but the plant-like Gonium has it in every one of its 

 zoospores. What, then, is there to distinguish the Gonium, 

 when in its young zoospore state, from the Chlamidomonas, 

 (p. 145, fig. 87,) which I described in another place, or still 

 less from the Euglena, (p. 144, fig. 86,) with its green contents, or 

 Heteromastix, Ceratium, Pleuronema, &c., which I have shown to 

 be closely related to each other (pp. 144 to 150) in an ascending 

 series from the zoospore-like Chlamidomonas to the highly or- 

 ganized Pleuronema 1 Indeed, some of the most recent inves- 

 tigators, like Messrs. Claparede and Lachman, have unhesitat- 



