CHAPTER XIII 

 PROTOZOA AND METAZOA 



WHEN hunting for Vorticella in sea or fresh water one fre- 

 quently comes across little branching colonies composed of 

 numerous Vorticella-like individuals united together to form 

 a sort of tree or bush. These belong to the genera Epistylis, 

 Carchesium, and Zoothamnium. The genus Epistylis may 

 readily be recognised, because the individuals forming the 

 colony have no contractile filament and therefore cannot 

 retract themselves by coiling their stalks. There is no organic 

 connection between the individuals ; they are merely associated 

 together and grouped in such a way as to form a plant-like 

 growth. In the genus Carchesium the individuals are similarly 

 associated to form a bush-like colony, but each has a well- 

 developed contractile filament, and can retract itself inde- 

 pendently of its fellows. A strong stimulus may cause all the 

 members of the .colony to retract themselves at once, but 

 generally some are retracted whilst the others remain fully 

 extended and expanded. In the genus Zoothamnium there 

 is a main stem with branches which again sub-divide so 

 that the colony has something the shape of an espalier pear- 

 tree. The contractile filament runs through the main stem, 

 and, sub-dividing, is continued into its branches, the ultimate 

 sub-divisions terminating in the Vorticella-like individuals. 

 Thus all the members of the colony are organically united by 

 means of the branched contractile filament, and a stimulus 

 affecting any one member is immediately communicated to all 

 the others, causing the whole colony to contract into a nearly 

 globular form. Further than this, in certain species of 

 Zoothamnium, the individuals of a colony are not all alike. 

 The greater number of them, forming the termination of 

 the twigs and branches, do not differ materially in structure 

 from Vorticella, and measure about o - o8 mm. in diameter. 

 Other individuals of much larger size, measuring as much as 



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