38 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



is able to withstand such vicissitudes as drying up for a considerable 

 period. The Copromonas is no doubt swallowed in this stage by the 

 frog or toad and passing through the alimentary canal is eventually 

 voided in the faeces. 



VOLVOX 



The genus Volvox owes its most conspicuous difference from the 

 flagellates hitherto described to a peculiarity in its method of fission. 

 When the cell-individual undergoes fission the resulting individuals 

 instead of separating and leading an independent existence remain 

 attached together by slender threads of cytoplasm so as to form a cell- 

 community or colony. 



The cell-community (Fig. 17, A) is spherical or slightly ellipsoidal in 

 shape. It consists of many, it may be several thousand, cell-indivicluals 

 and is large enough (up to nearly i mm. in diameter) to be distinctly 

 visible to the naked eye. Each cell-individual (Fig 17, B) is ellipsoidal 

 in form and is surrounded by a thick jelly-like envelope,, the envelopes 

 of the various individuals taking a prismatic form owing to their mutual 

 pressure and together constituting the thick wall of the colony in which 

 the cell-individuals are embedded. Each cell-individual possesses a 

 pair of flagella which project beyond the surface of the jelly into the 

 surrounding water, a pair of contractile vacuoles (Fig. 17, B, c.v) which 

 contract alternately, and a rounded nucleus (n). The greater part of 

 the cell, all except its outer end, is ensheathed in a thin green chromato- 

 phore and at one point embedded in the chromatophore near its edge is 

 a bright orange stigma (Fig. 17, B, st}. Finally each cell-individual is 

 connected with its neighbours by extremely delicate thread-like bridges 

 of cytoplasm (b). 



During life the projecting portions of the flagella perform active 

 lashing movements by which the colony is propelled along in a charac- 

 teristic manner which led an examination candidate to describe Volvox, 

 correctly if not very clearly, as a creature which Amoves in a 

 direction at right angles to that in which it goes." What happens is 

 that the Volvox colony rotates and at the same time advances along a 

 line which is roughly the axis of rotation. In other words the movement 

 is somewhat similar to that of a rifle bullet although not so regular. 

 This peculiarity in the movement of the Volvox community is correlated 

 with peculiarity of structure in the cell-individuals, for stigmata are 

 present only in the individuals situated in that hemisphere of the colony 

 which is in front as the colony moves, and further the stigma is situated 

 on the side of each individual which is in front. In other words the 



