156 GENEEAL HISTORY OF THE IXEUSOEIA. 



genera — as, for example, around the microgonidia of Cldamydococcus Avhen 

 they enter on their resting- stage, and about the cells of Istejjhanosjplicera 

 when preparing to leave the common envelope. But, fmiher, in these in- 

 stances, when this special closely apj^lied membrane appears, the envelope-cell 

 breaks uj) into a mucilaginous layer, and then presents the noimal condition 

 of that of Oonium. In other structural matters, in the number of vibratile 

 filaments, and in the history of development, Gonium entirely accords ^^'ith 

 the other genera. 



After this review of the affinity of Gomum with the other Volvoeinece, it 

 follows that, hke them, it must be of a vegetable natui'c, although cellulose 

 has not been detected in it. Still more, the evident relation of Gonium with 

 Pediastrum (II. 44), the plant-natiu'c of which no one at the present day ^vill 

 gainsay, points to the same natural position. It agrees with that plant in 

 general stnicture, in the union of several cells in one j)lane, in the number of 

 those cells and in their self-fission in the power of two, in the development 

 of new tablets and in obedience to the same laws. The only difference 

 between these two genera is, that in Pediastrum the swarming of the cells, 

 although sui'rounded by a common envelope- cell, ceases when they are asso- 

 ciated together in a tabular form, — whilst in Gonium the reverse is seen, the 

 power of motion becoming manifest when the several cells are in combination. 

 To state tliis generally : in Pediastrum the indi\idual cells swarra, and the 

 colony is quiescent ; in Gonium the colony swarms, and the quiescent state 

 of the several cells follows upon their separation. 



However, there are organs in Gonium which, did they admit of proof as 

 essentially animal structuiTS, would be fatal to all these arguments for its 

 vegetable natui^e. These are the two, or more rarely three, permanent 

 vacuoles visible near the ongin of the -vdbratile filaments, which are seen to 

 contract and expand alternately within a brief interval. These contractile 

 vesicles have a sharp outline, are colouiless, and look like clear rings in the 

 midst of the green cells. To detect them and their movements, the most 

 translucent and large cells must be chosen ; they must also be perfectly still, 

 and lie flat upon the glass shde, — an object attainable by a partial evaporation 

 of the di^op of water. 



The two vacuoles (XIX. 33) are but little apart, equally clear and large, 

 and apparently unconnected. Their action is alternate, each vacuole under- 

 going a systole and diastole in succession, whilst the time occupied by the 

 systole, by the diastole, and by the interval is equal. The same equality 

 in time obtains also between the two vacuoles of the same cell. Likewise a 

 uniformity prevails among the difi'erent cells of the same Gonium, but not 

 among the cells of different specimens ; and Cohn holds the occiuTcnce of 

 rhji:hmical contractions of these vacuoles as a well-established fact. 



These, therefore, are pulsating spaces, filling up T\ith water, and after a 

 time expelling it, and agree in all points with the so-called ' seminal vesicle ' 

 of Ehrenberg (the contractile sac or vesicle of other authors) met with in 

 ciliated Infusoria. Cohn next proceeds to discuss the question if these pul- 

 sating sacs are to be considered exclusively animal organs, and anives at the 

 conclusion that they cannot be so considered, and cannot be appealed to in 

 the decision of the question of the animal or vegetable natui'e of any doubtful 

 organism. 



To conclude this complete history of Gonium, as abstracted from Cohn's 

 elaborate essay, we must add that the description applies only to Gonium 

 ■pectorale (Ehr.), which, in the author's opinion, is the only species referable 

 to the Volvocinece, the remainder enumerated by Ehrenberg being members 

 of the genus Merismospedia of the PaJmellacece. 



