SERTULAEIA. 173 



the mode whereby the vesicle and its contents are generated is problema- 

 tical. When vigorous hydrse seem to originate from twigs rising through 

 the centre of vesicles, it might be presumed that the vesicle has origina- 

 ted on the orifice of a twig opening from or sustaining it ; or, that the twig 

 sprung up after the contents of the vesicle were discharged. 



After preservation for some time, the pith of the extremities exhibits 

 symptoms of decay, which, in descending, impairs the reproductive faculty. 



The hydrae are very susceptible of external impressions. I have ob- 

 served the whole of a specimen, amounting to an hundred, with the tenta- 

 cula closed up of a gloomy morning. 



Many circumstances concur in rendering these Sertulariae, whether 

 varieties or not, favourable for general observation. I doubt not that 

 other naturalists may ultimately show distinctions, which I have been un- 

 able to detect among them. 



Plate XXVII. Sertularia halecina. Adult specimen with green vesicles. 



Plate XXVIII. — Sertularia halecina. 

 Fig. 1 . Adult with yellow vesicles. 



2. Yellow planulse from the vesicles, enlarged. 



3. Two nascent Sertularise from the planulae, enlarged. 



4. Nascent Sertularia, farther advanced. 



5. Nascent Sertularia, still farther advanced, having three hydrae dis- 



played. 



6. Nascent Sertularia with three hydrse. 



7. Nascent Sertularia with three hydrae and a bud. 

 All the preceding, except fig. 1, enlarged. 



Plate XXIX. Sertularia Jialecina — details. 

 Fig. 1. Front of a young hydra, enlarged. 



2. Hydra protruding from a twig, with frills. 



3. Portion of a specimen with green hump or gibbous vesicles, en- 



larged. 



4. Extremity of a branch with vesicles. 



5. Ovoidal grey vesicles. 



