156 ZOOPHYTES. 



temperature of the atmosphere, — and especially when about to undergo 

 the metamorphosis incident to their race. 



One planula, fig. 4, having become quiescent, a spine rose from it, 

 which being sufficiently prolonged, exhibited two buds, almost mature, on 

 May 1. The planulse of the same brood were only advancing in the ve- 

 sicles on the 19th of April. 



The evolution of the nascent Sertularise, from vesicles in situ, is a 

 rare occurrence. We have seen that, from some unnatural retention in 

 the cysts of the Tubularise, the organs of the young may begin to unfold. 

 This may tend to corroborate and explain a figure given by Ellis, repre- 

 senting a hydra issuing from a vesicle of the Sertularia pumiln. But it is 

 to be noted also, that examples are not wanting of portions of the Sertu- 

 larise vegetating through an empty vesicle, vsdth a generated or regenerated 

 hydra. I can account for it only from the sudden metamorphosis fre- 

 quently rendering the planula motionless, and thus precluding its escape 

 from the vesicle. But although this may ensue in the Sertularia abietina, 

 the discharge of the planula from the vesicle, to undergo its metamorpho- 

 sis unrestrained, is the ordinary and natural course whereby the species is 

 perpetuated. 



It is doubtful whether the vesicle is a permanent part of the organi- 

 zation in any of the mature prolific Sertularise. Here the foundation of 

 the compound vesicle, PI. XXIV. figs. 3, 13, 14, b, becomes more and 

 more unstable, until at length its adherence to the branch appears to be 

 preserved only through the medium of adventitious matter. Did the 

 nascent Sertularia originate regularly in the vesicle, while subsisting, and 

 were it still retained there during the fall, its fixture would be intercepted 

 from some more solid foundation below, which could not but prove inju- 

 rious. Therefore, evolution of the hydra within the spherule of the com- 

 pound vesicle seems the result of unnatural retention. 



Young hydree have survived about a month. The Sertularia abietina 

 is not uncommon, but such specimens as that represented are rare. Some 

 consist of a greater number of subordinate parts. 



