MIMOSELLA. 377 



From a creeping fibre, which spreads over the surface of 

 Fuci, rise graceful, tapering stems, pinnate, much attenuated 

 towards their extremities, and running out into filamentary, 

 tendril-like prolongations. These stems are commonly from 

 an inch to an inch and a half in height. They are jointed 

 at intervals ; and immediately below each joint spring two 

 opposite pinnse, also jointed, tapering, and slightly curved. 



"The pairs of pinnse do not all lie in the same plane. 

 Along these are set the cells, which are ovate, elongate, 

 biserial, and opposite. Each cell is attached to a small 

 prominence on the side of the pinna, which is perforated. 

 A circular orifice on one side of the cell near the base fits 

 over this, and a joint is thus secured, by means of which 

 the polype can move its dwelling forward in one direction 

 and back again. This is frequently done. The polypes 

 are continually swaying their cells to and fro ; sometimes 

 all the cells on the pinna are folded together on the upper 

 side, just as the leaflets close on the leaf of the sensitive- 

 plant {Mimosa), and hence the generic name. Towards 

 the base of each pinna the cells are long and oval ; as they 

 approach the apex they became short and globose, and at 

 last are little more than little round excrescences. 



"The polypes have eight arms, and are furnished with a 



