40 OUTLINES OF EVOLUTIONARY BIOLOGY 



Paramcecium and many other Protozoa, again, have specialized 

 contractile threads of protoplasm lying just beneath the surface 

 of the body, comparable to the muscle fibres of higher animals, 

 which enable them to perform movements of a different kind 

 from those effected by flagella or cilia. By means of such con- 

 tractile fibres, localized mainly in a long stalk, the bell- 

 animalcule, Yorticella (Fig. 9, 10), can instantaneously draw in 

 its ciliated disc and pull itself out of harm's way on the approach 

 of danger." Other examples of these highly organized ciliate 

 Protozoa (Infusoria) are shown in Fig. 9. These forms should 

 be cjfcpared with the Radiolaria and Foraminifera represented in 

 Figs. 3 and 4. 



Another very important factor in the progress of organic 

 evolution is supplied by the principle of co-oDemtion between 

 different organic units. This is illustrated to some extent in the 

 process known as colony-formation met with in certain 

 Protozoa and Protophyta. Carchesium, Epistylis and Zootham- 

 nium (Fig. 9, 1115 ), for example, are colony-forming Infusoria 

 closely related to Yorticella. Yorticella itself multiplies rapidly 

 by simple longitudinal fission. The bell-shaped protoplasmic 

 body at the end of the stalk divides into two parts, one of which 

 swims away, attaches itself to some foreign object, and develops 

 a new stalk. If, instead of separating, the two daughter cells 

 remained together and went on dividing, and if this division also 

 extended each time to the upper part ofthe stalk, the organism 

 would presently arrive at a branching, tree-like condition. This 

 is what has happened in Carchesium, Epistylis and Zootham- 

 nium, and also in various other Protozoa. 



This arborescent type of colony-formation, moreover, is by 

 no means the only one met with amongst the Protista. It is 

 characteristic of stalked forms. Other forms, which are not 

 stalked, may give rise to free-swimming or floating, solid or 

 hollow aggregates of plate-like or it may be spherical shape. 

 The development of such colonies is perhaps nowhere better 

 seen than in the small group of unicellular organisms to which 

 Hsematococcus belongs, the members of which are known, on 

 account of their plant-like character and their possession of 

 flagella, as Phytoflagellata. 



Hsematococcus, or the closely related Chlamydomonas, may be 

 taken as the starting point of the series. These two are always 

 solitary, the individuals separating completely from one another 



