BICELLIRIA. 67 



but it is distinguished by grace, delicacy, and picturesque- 

 nesSj the latter being chiefly due to the great development of 

 spines, which is a feature of the genus, and to the peculiar 

 turn of its cells in most of the species. The aperture varies 

 in the proportion which it bears to the length of the cell, 

 and the degree in which it is turned inwards. Occasionally 

 it is subterminal, and looks upwards much more decidedly 

 than is usual. In one or two Australian species it is 

 elongated, and veiy much bent inwards towards the 

 median line. In our own B. Alderi, while there is the 

 same general form, the cells are much straighter, and the 

 arrangement is more compact and regular, reminding us, 

 as I have already remarked, of the Cellulariida. 



The spines are profusely developed in this group, and 

 occur not only on the margin of the aperture, but also 

 on the dorsal surface. The avicularia, when present, 

 are but sparingly developed ; and in some species they 

 are altogether wanting. In this respect Bicellaria offers 

 a contrast to Bugula^ which is remarkable for the pro- 

 fusion of these appendages as well as for their highly 

 specialized form. 



As in Caberea, there is in some of the species a wonder- 

 ful development of the radical fibres. In the exquisite 

 Australian form, B. tuba, Busk, which grows in feathery 

 tufts of considerable size, they extend along the back of 

 the shoots for about two thirds of their height, forming a 

 compact mass of white, glistening tubes. Each tube ori- 

 ginates on the back of a cell, and, so far as I have observed, 

 of those particular cells only which are placed between the 

 diverging branches at a bifurcation. From these points 

 or stations the fibres tend downwards, uniting as they pro- 

 ceed with the bundle already formed below them, which 

 at the base resolves itself into a multitude of separate 

 rootlets. 



f2 



