Originally produced from a single ovulum, every species, by the evolution 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 uncertain 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. 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 constitute a family whose workings are all regulated by one harmonious 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 cœlenterates we find a similar organisation. All the soft parts of a sertularian polypary are enclosed in a horny sheath (_hydrosoma_) which develops peculiar cup-shaped processes (_hydrothecæ_) for the protection of each individual polyp, and capsules for the reproductive bodies (_gonoblastidia_) in which the ova are produced. The various modifications of form and structure of the polyps, of their hydrothecæ and gonoblastidia, give rise to a number of families, genera, and species. Thus in the Sertulariæ the polypites are sessile, biserial, alternate, or paired; sessile and uniserial in the Plumulariæ, and stalked in the Campanulariadæ. [Illustration: _a._ _Laomedea neglecta_, natural size. _b._ Portion of the same, magnified. _c._ Reproductive body of _Campanularia volubilis_. _e._ Reproductive body of _C. syringa_. ] The free-swimming Jelly-fishes, or Acalephæ, 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 the sight; but these shapeless objects were beautiful while they moved along in their own element, and their simple organisation shows no less the masterhand of the Creator than the complex structure of the higher stages of animal existence. With the exception of the Ctenophora, they all belong to the hydrozoic class, and from the great diversity of their structure have been ranged under four orders, Medusidæ, Lucernaridæ, Calycophoridæ, and Physophoridæ. [Illustration: