_a_, _e_, _f_. Threads and thread-cells of _Caryophyllia Smithii_. _b._ Thread-cell of _Corynactis Allmani_. _c._ Peculiar receptacle of _Willsia stellata_, containing thread-cells. _d._ A single thread-cell of the same. _g._ Thread-cell of _Actinia crassicornis_.--(All magnified.) ] The Cœlenterata have been subdivided into two great classes: the Hydrozoa, in which the wall of the digestive sac is not separated from that of the cavity of the body, and the Actinozoa, in 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 cœlenterate life, some are of a compound nature (Sertularidæ, &c.), and, having once settled, remain permanently attached to the site of their future existence; while others (Rhizostomidæ, &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 Medusæ. 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 Flustræ, they might easily be mistaken for sea-weeds. [Illustration: Sertularia tricuspidata. _a._ Skeleton (natural size). _b._ Portion of the same, highly magnified. _κ._ _Cœnosarc_, or common trunk. _π′._ _Hydrotheca_, or protective envelope of individual polyp. _ρ′._ _Gonoblastidium_, or reproductive germ or body. ]