534 Annals of the South African Museum. 



one coenobium, or adult rotifers may be watched eating their way 

 into a colony from the outside. A large proportion of the somatic 

 cells may be eaten away before the colony ceases to move, while the 

 developing embryos are more or less completely destroyed, or else are 

 malformed and distorted owing to the activities of the destructive 

 little parasites during the course of their development. 



While in the case of the amoeboid parasite low temperature and 

 high hydrogen-ion concentration seem to be predisposing causes of 

 susceptibility to attack, in the case of the rotifer long-continuance 

 of Volvox in the pool in question would appear to favour the develop- 

 ment of the parasite, which is rarely seen early in the season. During 

 the subsequent appearances of Volvox as the season advances, 

 however, it becomes increasingly abundant. This has been noted 

 in several vleis, particularly Scanlan's Vlei (Wetton Road) and 

 Vlei III, near Belvedere Road. 



The almost complete absence of the rotifer from the Kimberley 

 Volvox one or two individuals were noticed in some of the last 

 collections is confirmatory of this, since there the duration of the 

 pool after each rain was very brief, and normally the pool dried 

 completely between successive phases. 



The crustaceans found with Volvox do not appear to have much 

 power of attacking the uninjured colonies, but may apparently feed 

 to some extent on coenobia which have been distorted or torn in some 

 way. Very occasionally fine fungal hyphae were observed in the 

 mucilaginous content of the hollow spheroids. 



When badly parasitised, Volvox shows the effect macroscopically 

 by the pale yellowish-green colour of the colonies ; if this is due to 

 the activity of rotifers, peculiarly dark opaque bodies (the eggs of 

 the rotifer) inside the colonies may be distinguished from the young 

 daughter colonies by the naked eye. 



Macroscopic Observations. 



If active living material of Volvox is placed in a clear glass tube 

 and held up to the light, a number of features are distinguishable 

 by the naked eye. In a quantity of a well-advanced strain of 

 F. Rousseletii so examined, the following details may be seen : 



The colonies are a bright, clear leaf-green, swimming about actively, 

 rotating about a polar axis which is perceptibly greater than the 

 equatorial diameter in the older colonies, although the smaller ones 

 appear spherical. The pole which is directed forward as the colony 

 moves is lighter in colour than the posterior half, in which in addition 



