408 W. ATRTON ON ZOOTHAMNJUM GENICVLATVM. 



Seen with the naked eye or pocket-lens, the whole colony of 

 Zoothamnium geniculatura appears very similar in size and shape 

 to that of Zoothamnium arhuscula^ though anyone well acquainted 

 with both species will easily discern a difference in the fern-like 

 fronds and mode of contraction. 



The chief difference from other species, and peculiarity, is in 

 the main stalk or pedicle. This can be divided into three 

 regions ; the upper part is flexible, and contracted by the internal 

 muscular thread into a more or less spiral form, while the two 

 lower parts are perfectly stiff, but connected by a highly flexible 

 knee-joint (Figs. 4 and 7). In contraction this knee-joint is the 

 most remarkable and obvious feature, and has suggested the 

 specific name. At the summit the pedicle divides horizontally or 

 obliquely into from five to nine fern-like fronds, with bipinnate or 

 alternate branching secondary stalks, which are again subdivided 

 as represented in Figs. 2, 3, and 8. The nodes are close together, 

 and two or three smaller branchlets usually spring from the 

 underside of each node. In Zoothamnium arhuscula the second- 

 ary stalks of the fronds are fine, smooth, and elongate, while in 

 Zoothamniura geniculatum they are stout, short, and knotty (Fig. 8). 

 All the ultimate branchlets and zooids are situated on the under 

 side of the frond, and the current in the water, produced by the 

 total action of the cilia, runs from below the tree upwards and 

 through the branches. 



The zooids are of two shapes and sizes. The very numerous 

 smaller zooids are conical-campanulate and of the usual Vorticella 

 structure (Fig. 3). The larger reproductive zooids are four times 

 as large when fully grown, spherical in shape, pellucid, with a 

 very small ciliated cup at the apex, and contain a large crescentic 

 nucleus of a bluish colour in their interior ; they are always few 

 in numbers and attached to the underside of the main rib of the 

 fronds (Fig. 6). 



Attached to the branches I have noticed parasitic Vorticellae 



