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On Zoothamnium geniculatum, a New Infusorian. 



By William Ayrton. 



(^Bead December I9th, 1902.) 



Plate 21. 



In the summer of 1899 I first observed this distinctive tree-like 

 colony-stock of the Vorticella tribe, attached to weeds growing in 

 the river Waveney, near Beccles, Suffolk. On one occasion, in 

 November or December of 1899, 1 brought some specimens to a 

 meeting of the Quekett Club, and the opinion was then expressed 

 that it was Zoothamnium arhuscula or a form closely allied to it. 

 Further study and observation have brought to light very 

 distinctive features, which make it evident that we have to deal 

 with a new and undescribed species. 



There are four genera of the Vorticellidae forming compound 

 tree-like colony-stocks — namely, Opercularia, Episti/lis, Carchesium 

 and Zoothamnium. In the first two the pedicle and branches, or 

 zoodendrium, are rigid and uncontractile. In Carchesium the 

 zoodendrium is contractile, but the central muscular fibrilla is 

 disconnected, and each zooid or group of zooids is able to contract 

 separately. In Zoothamnium the muscular band is continuous 

 throughout, so that when a single zooid contracts, all the others 

 follow suit, and there is a complete contraction of the whole 

 colony-stock. 



