The Physophoræ have no hydrophyllia, but their swimming-bells are considerably developed, and serve as powerful instruments of locomotion. They are also provided with certain processes termed "hydrocysts," which some observers appear disposed to regard as organs of touch. Such are but a few of the numerous genera of the Physophoridæ. [Illustration: Physophora Philippii. α. Pneumatophore. ν. Swimming-bells. φ. Hydrocysts. π. Polypites. τ. Tentacles. ] Of the jelly-fishes in general it may be remarked that, though they are denizens of the frigid as well as of the temperate and tropical seas, their beauty increases on advancing towards the equator, for while the Medusæ in our latitudes are generally dull and obscure, those of the torrid zone appear in all the splendour of the azure, golden-yellow, or ruby-red tints which distinguish the birds and fishes of those sunny regions. They are indeed of no immediate use to man, but their indirect services are not to be despised. They partly nourish the colossal whale, and thus, converted into oil, attract thousands of hardy seamen to the icy seas; numberless Crustacea and molluscs also live upon their hosts, and are in their turn devoured by the mighty herring shoals, whose capture gives employment and wealth to whole nations of fishermen. [Illustration: Development of Chrysaora hysoscella. _a._ Ova with gelatinous investment. _b_ and _c_. Free ova. _d._ Young Hydratuba developed therefrom. _e._ The same with eight tentacles. _f._ Hydratuba in its ordinary condition. _g_, _h_. More advanced forms, with constrictions. _i._ A specimen undergoing fission, in which the tentacles are seen to arise from below the constricted portion, while its upper segments separate and become free-swimming zoöids (_k_). ] Armed with that wonderful instrument, the microscope, naturalists have been taught to disunite in many cases animals which from their external resemblance were formerly supposed to belong to the same class or family; and to join others to all appearances extremely dissimilar. Thus the Bryozoa have been detached from the polyps, in spite of their similitude of growth, while the roaming and fixed Hydrozoa have been found in many cases to be but alternating generations or various phases of development of the same animal. Take, for instance, _Chrysaora hysoscella_ (see preceding figure, page 351), one of our commonest jelly-fishes. The ova this free-swimming creature produces might naturally be supposed to develop themselves into equally free-swimming Chrysaoræ; but instead of this they soon become attached, and grow into a colony of sessile Hydratubæ, as, at this stage of their career, they have been termed. For years they may thus continue, but then the evolutions shown in the annexed illustration take place until free-swimming zoöids are detached, which eventually become similar to the huge Chrysaora, from one of whose ova the primitive hydratube was produced.