VOLVOX. 



[ 682 ] 



VOLVOX. 



Solitary. 



Without cilia Gyges. 



With a pair of cilia CMumidomonus f. 



Grouped. 



Forming a square layer, indivi- \ Pn .,.. > 

 duals with two cilia / GoNIUM ' 



Forming a spherical body. 

 Cilium solitary. 



With a " tail." $ .&.J.V.V Uroglena *. 

 Without a " tail." 



Without an eye-spot. 

 Without special coats . Pandorina *. 

 With special coats .... Svncrvnta *. 



With special coats. 



With an eye-spot. 



Individuals dividing \ Eudorina * 

 into two .......... J 





__ 



Cilia twoT" 



Without an eye-spot. . . . Synura * . 

 With an eye-spot. 



* Probably stages of development of VOLVOX. 

 t Zoospores of PROTOCOCCUS? 



The names in small capitals are well-esta- 

 blished genera. 



BIBL. See the genera. 



VOLVOX, L. A genus of Volvocinese 

 (Confervoid Algae), of which only one species, 

 V. globator (PI. 3. fig. 24), seems satisfac- 

 torily established. This organism, occurring 

 not uncommonly and often in great abun- 

 dance in clear pools on open commons, &c., 

 appears to the naked eye as a minute pale 

 green globule gently moving about in the 

 water, its dimensions variable, but generally 

 about 1-50" when full-grown. When placed 

 under a low magnifying power it is found to 

 be a spherical membranous sac, studded all 

 over with green points, the entire body rolling 

 over in the water with a motion which is 

 readily discerned to be caused by innumera- 

 ble cilia arranged upon the surface of the 

 globe. In the interior of the sac are gene- 

 rally seen dense globes, in summer mostly of 

 green colour (PL 3. fig. 24); sometimes the 

 cavity is wholly filled up by a number of 

 membranous sacs exactly resembling the pa- 

 rent, but deformed by mutual pressure (PL 3. 

 fig. 25), and inside these are seen smaller 

 green bodies, as in the former case. The 

 parent envelope is also flexible, yielding to 

 pressure and recovering its form, and in full- 

 grown specimens is generally ruptured at 

 one point, where the internal bodies escape, 

 so that the number varies; usually, how- 

 ever, the original number is eight. 



The application of higher powers is requi- 



site to discover the intimate structure of 

 Volvox, which, by the researches of William- 

 son and Busk, most of whose observations 

 we have verified, has been pretty clearly 

 made out. The outer envelope consists of 

 a layer of cell-membrane, in all probability 

 composed of a modification of cellulose, al- 

 though we have never succeeded in producing 

 more than a faint purple tinge with sulphuric 

 acid and iodine. By the application of a 

 sufficient magnifying power, the green cor- 

 puscles at the periphery are found to consist 

 of zoospore-like bodies (PL 3. fig. 28), which 

 are seated inside the membranous envelope, 

 each sending out its pair of vibratile cilia 

 (figs. 24-30) through separate orifices in the 

 external coat. The same investigation will 

 reveal that the green corpuscles have radia- 

 ting processes extending from their sides, 

 and running from the different centres to 

 meet each other in the light interspace, 

 forming thus a kind of delicate network be- 

 neath the membrane. The green corpus- 

 cles are pyriform, have a transparent anterior 

 end bearing a pair of cilia, and contain a 

 reddish-brown eye-spot and a contractile 

 vacuole, thus exactly resembling those of 

 Gonium, and indeed the zoospores of Con- 

 fervoids generally. The radiating processes 

 resemble those found in particular stages of 

 PROTOCOCCUS pluvialis, running through 

 the gelatinous coat, and probably may be 

 compared with the radiating filaments pro- 

 ceeding from the nucleus of SPIROGYRA' 

 (PL 5 fig. 26). There is somewhat more 

 difficulty in determining the nature of the 

 structure in which the green corpuscles are 

 enclosed. There is a layer of soft consistence 

 of some thickness within the external mem- 

 brane ; the green corpuscles are wholly im- 

 bedded in this, and their radiating processes 

 and cilia traverse the substance of it. We 

 are inclined to believe that this presents a 

 firm membranous layer again at the internal 

 surface, looking toward the general cavity of 

 the sphere. The nature of the soft layer 

 has been the subject of discussion ; we be- 

 lieve Busk's view to be correct, that it is not 

 formed by the collocation of distinct mem- 

 branous cells, like those of ordinary paren- 

 chymatous structures ; but by the close 

 juxtaposition of gelatinous envelopes of the 

 individual green bodies, resembling those of 

 Coccochloris, Glceocapsa, &c. We could 

 never detect a true line of demarcation half- 

 way between neighbouring green corpus- 

 cles; an appearance is indeed sometimes 

 presented in preparations kept in chloride 



