Protozoans 



161 



Cothurnia (fig. 73c) 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 

 colonies lie scattered about over the 

 bottom of a lake or pond. They are ^^^'^Is'an 

 roundish, or often 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 verv different from the fore- 



FlG. 



72. A colony of 

 Epistylis. 



egg, probably the egg of 

 a rotifier. 



