THE SERTULARLE. 347 
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 ccelenterate life, some are of a compound 
nature (Sertularida, &c.), and, having once settled, remain per- 
manently attached to the site of their future existence; while 
cthers (Rhizostomide, &e.) 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 
Medusz. 
The sertularian tribes 
are remarkable for the 
elegance of their forms, 
resembling feathers more 
or less stiff and angular, 
mere 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 
Flustre, they might 
easily be mistaken for 
sea-weeds. 
Originally — produced 
from a single ovulum, 
every species, by the 
evolution of a succession 
of buds, after an order Sertularia tricuspidata. 
eculiar to ea o a. Skeleton (natural size). 5. Portion of the same, highly 
se ch, ShOWS magnified. xz. Caenosarc, or common trunk. 2’. Hydro- 
up toa populous colony, theca, or protective envelope of individual polyp. 
e’. Gonoblastidium, or reproductive germ or body. 
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 rout fibres are 
full of the same medullary substance with the rest of the bod ye 
