GENERAL MORPHOLOGY 



19 



or a Bursaria truncatella, reaching the size of 2 mm. (one-twelfth of an 

 inch), while the parasitic gregarine Porospoi-a gicjaniea of the lobster's 

 gut attains the length of 16 mm., or two-thirds of an inch. 



Unlike the majority of bacteria, the size of any given species of 

 protozoa often varies within wide limits, and this in the same environ- 

 ment. The reasons for this difference are numerous, sometimes it is 

 due to starvation, sometimes to developmental condition, and some- 

 times to the variations in vitality at different periods in the life history. 

 Thus, two cells from the same culture of dileptus species may be mis- 

 taken for different species, the difference between them being so great, 



Fig. 2 



Dileptus, sp. Two sister cells. A, normal individual with macronucleus in form of 

 scattered chromatin granules (chromidia); B, individual starved for several days. From 

 photographs taken with same magnification. 



and due solely to the lack of food in one case (Fig. 2). This 

 divergence in size is particularly noticeable in the parasitic forms, 

 where many factors influence the development of the cell. 



Many forms of protozoa, especially the flagellated types, have 

 acquired the habit of association into colonies, and with such associa- 

 tion have gained the economy which comes from division of labor, 

 so that here in the colony forms may be found the first step in the 

 differentiation of cell aggregates and the nearest approach of protozoa 

 to the metazoa. Such colonies have been designated according to 

 their mode of formation, gregaloid, spheroid, arboroid, and catenoid 



