348 THE INHABITANTS OF THE SEA, 
Thus the graceful sea-fir (Sertularia cupressina), the largest of 
our native species, may attain a height of two or three feet, and 
bear on its branches no less than 100,000 distinct microscopical 
polypi, each with its own crown of tentacles, and each of. these 
armed with numerous thread-cells, as formidable in their way as 
the crustacean’s claw or the annelide’s embrace. But though 
each polyp has a certain share of independence yet its body is 
continuous with the more fluid pulp that fills the branches and 
stem of the common trunk, and by this means all the polyps of 
it are connected together by a living thread, and made to con- 
stitute a family whose workings are all regulated by one har- 
monious instinct. Each of 
these plant-like structures 
may therefore be considered 
as one animal furnished with 
a multitude of armed heads 
and mouths, and in all the 
other compound ccelenterates 
we find a similar organisation. 
All the soft parts of a sertu- 
larian polypary are enclosed 
in a horny sheath (hydro- 
soma) which develops peculiar 
cup-shaped processes (hydro- 
thecew) for the protection of 
each individual polyp, and 
capsules for the reproductive 
bodies (gonoblastidia) in 
Precis tee erence reai winiely the ova are —prodMeei 
g Reproductive boar of Compenularia slab. The various modificaliongydl 
form and_ structure of the. 
polyps, of their hydrothece and gonoblastidia, give rise to a 
number of families, genera, and species. Thus in the Sertulariz 
the polypites are sessile, biserial, alternate, or paired ; sessile and 
uniserial in the Plumularie, and stalked in the Campanulariade. 
The free-swimming Jelly-fishes, or Acalephz, as they have 
been named by Aristotle on account of the stinging properties 
due to their urticating cells, are likewise among the commonest 
objects left upon our shores by the retreating tide. When 
stranded, they appear like gelatinous masses, disgusting to 
