SERTULARIA. 169 



The profusion of nascent Sertularise is sometimes very great. Above 

 300 were rooted on the bottom of a small vessel in a preceding year, on 

 the 5th of February, from green specimens. 



Facts of difficult explanation occur in the history of the Sertularia 

 halecina, such as regenerating bydrffi shooting up through the empty vesicles, 

 PI. XXIX. figs. 28, 29. Examples of this occurred in April. Some of the 

 vesicles on specimens were empty ; one or two hydrse were displayed from 

 others. These must have issued from the pith of their respective twigs, 

 which had certainly vegetated through the vesicle, from the stem, after its 

 formation, and probably after having discharged its contents. It is not un- 

 likely that some generation or regeneration of this kind may have induced 

 naturalists to credit the developement of hydrae in the vesicle, which, if it does 

 ensue, is only by deviation from the natural course, as signified previously. 



2. Among the cognates of the Sertularia halecina is one designated 

 by Dr Johnston Thoa Beanii. Lamouroux seems to have discarded the 

 Halecina from its place among the Sertularice ; and, if I understand his 

 work, to have constituted a new genus of it named Thoa, comprehending 

 two species. 



This, the Thoa Beanii, rises three inches or more, by a stem com- 

 posed of aggregated tubuli, together with boughs, branches, and twigs, all 

 like the former, diverging on each side in alternate arrangement, but 

 sometimes irregularly distributed. — Plate XXXI. fig. 1. 



The adult is of a brown or olive colour ; young specimens are white. 

 The hydra of older specimens is green or greenish ; but if the specimen be 

 young it is white, as also from parts newly generated. It has 20, 22, or 

 24 muricate tentacula, for the number is not uniform ; and when the pro- 

 duct is in greatest perfection the hydra issues from the extremity of a tu- 

 bular twig, having from two to seven frills, like those parts recently de- 

 scribed. But on the same specimen may be sometimes seen twigs both 

 without frills and with them. They are quite transparent in very small 

 specimens, and always prominent where present. — Plate XXXI. figs. 2, 3. 

 The hydra extends far from the orifice of the tube, when enjoying the 

 freshness of its element, and retreats partially within if annoyed. There is 

 here no proper cell. 



VOL. I. Y 



