SERTULARI2E. 121 



produce bag-like bodies, called vesicles, in which their 

 ova are contained. These are very rarely found on 

 S. filicula, but may be seen 

 abundantly, especially in 

 early spring, on another com- 

 mon species (S. oper ulata), 

 which frequently forms a 

 rigid beard to the stems of 

 the great Oar-weed. The form 

 of the vesicles varies much 

 in different kinds, and often 

 affords an excellent character 

 to distinguish one closely- 

 allied species from another. 

 In Plumularia cristata, a PORTION. 



beautiful feathery species, 



common on sea-weeds near low-water-mark, especially 

 on Halidrys siliquosa, the vesicle is exceedingly curious, 

 seemingly formed by the union and metamorphosis of 

 several cells. It consists of an oblong pouch, with a 

 tubular rib along its dorsal margin, from which issue 

 numerous transverse, crested ribs, which will be better 

 understood by the annexed figure than by a more detail- 

 ed description. The genus Plumularia is readily known 

 from Sertularia by having its cells unilateral, or all 

 placed along one side only of the branches. Speaking of 

 Plumularia cristata, Dr. Johnston introduces some re- 

 flections which apply equally to most of the Zoophytes 

 of this division, and which I shall therefore quote. 

 Each plume has been calculated to contain about five 

 hundred Polypes, and a single specimen of ordinary size 



