164 ZOOPHYTES. 



issued from tubular cells, at the end of the twigs, which were encircled by 

 a succession of ruffs or frills at intervals. 



All the preceding specimens were laden with vesicles. 

 The common features of the product seem a compound stem of tubuli ; 

 subordinate parts in alternate arrangement rising at an acute angle with 

 the principal parts ; hydrse issuing from tubular cells terminating twigs 

 distinguished by successive frills. It has always appeared doubtful, after 

 innumerable observations, whether the hydra, being once extruded from 

 the tubular extremity of the twig, preserves the faculty of again retreating 

 completely within. 



During earlier stages the stem is waved, and then, as in new acces- 

 sions, the subordinate parts rise from the salient angles. 



The discrepant aspect of different specimens is so great, as to occasion 

 much embarrassment in determining the identity of species, or of conclud- 

 ing them varieties. 



In endeavouring to explain the formation of this Sertularia, it must 

 be observed, that the succession of ruffs or frills seen in the finest speci- 

 mens, is not to be held as a definite character, especially if assumed as of 

 a precise number. The hydra appears to be regenerated more than once 

 from the same twig, and it is probable that each regeneration is accom- 

 panied by a circular enlargement, as a frill, of the orifice of the cell. Cer- 

 tain parts of a small specimen having been vacant of hydrse on the 5th of 

 April, had generated about a dozen on the 12th, besides others advancing. 

 —Plate XXX. fig. 8, enlarged. 



Under favourable circumstances, the progress of such hydrae is well 

 exposed through the wide transparent sheath inclosing the pith, fig. 9, 

 where the head appears like a compact, solid substance at the extremity ; 

 then altering its shape gradually, until completely displayed, fig. 10. It is 

 now discovered that the pith issues from the branch, and that the orifice 

 of the first cell is under the disc of the hydra. If there was one from 

 each cell previously in fig. 10, the hydra must have been close to the 

 branch. The nature of such frilled intervals is also very explicit from 

 Plate XXIX. fig. 2. 



