THE SERTULARLE. 



3iT 



which the stomach forms a distinct bag separated from the wall 

 of the cavity of the body by an intervening space, subdivided 

 into chambers by a series of vertical partitions. Each of these 

 two classes comprises a number of families of various forms and 

 habits of life. Thus among the Hydrozoa, with whom I begin 

 my brief survey of coelenterate life, some are of a compound 

 nature (Sertularidse, &c), and, having once settled, remain per- 

 manently attached to the site of their future existence; while 

 others (Rhizostomidse, &c.) continue freely to roam through the 

 water, and others again appear in the various stages of their 

 development either as sessile polyps or as free-swimming 

 Medusae. 



The sertularian tribes 

 are remarkable for the 

 elegance of their forms, 

 resembling feathers more 

 or less stiff and angular, 

 more or less flexible and 

 plumose'. Their bleached 

 skeletons are among 

 the commonest objects 

 thrown out by the waves, 

 and so plant-like is their 

 appearance and manner 

 of growth that, like the 

 Flustrse, they might 

 easily be mistaken for 

 sea-weeds. 



Originally produced 

 from a single ovulum, 

 every species, by the 

 evolution v of a succession 

 of buds, after an order 

 peculiar to each, grows 

 up to a populous colony, 

 and simultaneously with 

 its growth the fibres by which it is rooted extend, and at un- 

 certain intervals give existence to similar bodies, whence new 

 polypiferous shoots take their origin, for these root fibres are 

 full of the same medullary substance with the rest of the body. 



Sertularia tricuspidata. 



a. Skeleton (natural size), b . Portion of the same, highly 



magnified. », Ccenosarc, or common trunk, rr'. Hydro- 



iheca, or protective envelope of individual polyp. 



g'. Gonoblastidivm, or reproductive germ or body. 



