HISTORY OF SERTULARIA POLYZONIAS. 125 



lower ramifications; nevertheless., the persistence of 

 vigorous animated extremities may be long, and their 

 reproductions numerous, while all the lower ramifica- 

 tions are quite dead. 



Let us, however, illustrate the economy of these 

 beautiful organisms by an example. 



SERTULARIA POLYZONIAS. The Hoop Sertularia. 



Specimens of this zoophyte rise to about 4 inches 

 high, by a stem which is slightly waved, and scantily 

 provided with boughs and branches diverging to right 

 and left. The cells are ranged alternately on both 

 sides of the stem, the subordinate parts always ori- 

 ginating from the convexities by which these are 

 distinguished (PL II. fig. 6). 



Each cell is occupied by a Hydra furnished with 

 about twenty-four or twenty-six tentacula (PI. II. 

 fig. 6,a). The Hydra is large in proportion to the 

 size of the cell, from which it protrudes by means of 

 a long neck or body ; and, when disturbed, retreats 

 completely within the cavity. The whole zoophyte is 

 of a green colour, of various shades and intensities, 

 this colour being derived from the pith, as the horny 

 sheath is perfectly transparent. The stem of adults 

 or older specimens is occasionally yellowish or brown. 



The ovaria or reproductive vesicles of this Sertu- 

 larian are large in comparison with the cells, and of 

 singular structure, appearing as if they were composed 

 of hoops or belts united together (PL II. fig. 6, b). 



Specimens with lively Hydne, and bearing repro- 

 ductive vesicles, were obtained by Sir John Dalyell in 

 July. Only one vesicle appeared among twenty cells. 



