Protozoans 



i6i 



Cothurnia (fig. 73^:) is a curious double form that is 

 often found attached to the stems of water weeds. The 

 two cells of unequal height are surrounded by a thin 

 transparent lorica. For beauty of form and delicacy or 

 organization it would be hard to find anything surpas- 

 sing this little creature. 



Vorticella and its allies are among the commonest 

 and most ubiquitous of protozoans. They are sessile 

 and stalked, with some portion or all of the base con- 

 tractile. Vorticella forms clusters of many separate 

 individuals, while Epistylis forms branching, tree-like 

 compound colonies (fig. 72). Oftentimes they com- 

 pletely clothe twigs and grass stems 

 lying in the water, as with a white 

 fringe. Often they cluster about the 

 appendages of crustaceans and insects, 

 or thickly clothe their shells. Some- 

 times they cling to floating algal fila- 

 ments in the water-bloom (see fig. 1 79 

 on p. 295). 



Ophrydium forms colonies of a very 

 different sort. Numerous weak-stalked 

 individuals have their bases imbedded 



in a roundish mass of gelatin. The pio. 72. A colony 



of 



Epistylis. 



egg, probably the egg of 

 a rotifier. 



colonies lie scattered about over the 



bottom of a lake or pond. They are The dark object on the 



*^ -^ side of the stalk is an 



roundish, or 01 ten rather shapeless 



masses varying in size from mere specks 



up to the dimensions of a hen's egg. In the summer of 



1906 the marl-strewn shoals of Walnut Lake in Michigan 



were so thickly covered that a boat-load of the soft 



greenish -white colonies could easily have been gathered 



from a small area of the bottom. 



Other forms of protozoa there are in endless variety. 

 We cannot even name the common ones here : but we 

 will mention two that are very different from the fore- 



