172 ZOOPHYTES. 



The colour of the vesicle is derived from its embryonic contents — 

 developing as planulis, which, expelled from above, crawl away on reach- 

 ing the bottom of the vessel below. 



Prolific vesicles continue discharging planulae during two or three 

 weeks. 



The planula, like that of other species, becomes motionless, and con- 

 tracting in a circular spot, a spine rises from the centre, at the summit of 

 which a hydra is advancing, while the root, diifiising below, breaks into 

 divisions. This may be very distinctly seen by providing a watch-glass for 

 reception of the planula, and reversing it as the course of metamorphosis 

 proceeds. Sometimes the young Sertularia is seen floating inverted, with 

 the root upwards, while in a very early stage, from having been acciden- 

 tally detached in the commencement of metamorphosis. 



The original spine extending as a stem, seems to consist of a single 

 tube, very wide, formed like a skin or sheath, around the internal pith, 

 when we may plainly discover that the extremity from whence the hydra 

 issues has no cellular enlargement. — PI. XXX. fig. 7. 



The hydrae are developed progressively ; some being always less ad- 

 vanced. They are displayed originally without any definite number of ten- 

 tacula, such as may be distinguished as the complement of the race. 

 Either 17, 18, or 20, for the most part belong to them. In fig. 7, as 

 above, which is not so regular as many others, the one hydra had 17, the 

 other 20. 



An example occurred of the nascent zoophyte showing three hydrae 

 within nine or ten days of its production, as an active planula, from the 

 vesicle. 



Although originating Sertularise follow the escape of the planulae, by 

 the natural process, whole colonies disappear without such a result : and ' 

 they may resolve into the granulated matter above alluded to, concomi- 

 tant, likewise, on the dissolution of Planariae, or of some other semi-ge- 

 latinous animals. 



Disturbing the planulae seems to impair the process of effectual meta- 

 morphosis. 



Prolific vesicles and living hydrae appear on the same specimen, but 



