W. AYRTON ON ZOOTHAMNIUM GBNICULATUM. 409 



with slender spiral stalks, resembling Vorticella campanulata^ 

 ^i-Q in. in size, living among the zooids of Zoothamnium 

 geniculatum. A Rotifer also — probably Proales petromyzon — is 

 found living and laying its eggs on the branches, and feeding on 

 the zooids. 



The muscular fibre is continuous throughout all the branches, 

 and at the summit of the main pedicle the various threads run 

 together to form a stout muscular band (Fig. 5), which runs 

 down the stem to a point some little distance above the knee ; 

 from this point the band becomes very transparent, as if only 

 the sheath of the muscular thread were continued, and just below 

 the knee it is attached to the side of the pedicle (Fig. 7). The 

 muscular thread therefore does not run down to the base of the 

 pedicle, as is the case in Zoothamnium arhuscula. In structure the 

 internal muscular fibrilla consists of a cord with fine transverse 

 striations encased within a delicate hyaline sheath, which itself 

 is very finely striated longitudinally. 



The size of the whole adult zoodendrium is \ in. in height 

 and equally wide across the crown. The longest diameter of the 

 small zooids is ^\^ in. long and yi^ in. wide. The large 

 spheroidal zooids vary in size from -^^^j in. to ^\^ in. in diameter. 



The whole colony-stock of this infusorian presents the appear- 

 ance of a most beautiful living miniature tree in which the small 

 zooids represent the leaves and the large reproductive zooids the 

 fruit, and it is quite visible to the naked eye. 



The only habitat known so far is in the river Waveney, above 

 Beccles, in fresh water, where it is "to be found attached to 

 submerged weeds, sometimes in fair abundance, during the 

 summer months and even up to December ; but even there it 

 disappears sometimes for several years, and it must therefore be 

 considered a rare species. 



I have worked the other Norfolk rivers and broads for some 



