[Illustration: Various forms of Coryniadæ. _a_ and _b_. _Vorticlava humilis._ _c._ Four polypites of _Hydractinia echinata_, growing on a piece of shell. _d._ Portion of _Syncoryne Sarsii_, with medusiform zoöids (ρ), budding from between the tentacles (τ) of the polypite (ο). --(All, except _a_, magnified.) ] In a similar manner the Coryniadæ, a family of hydrozoic polyps, which, unpossessed of the firm investment of the sertularians, are frequently found decking sea-weeds and stones with dense arborescent structures, give birth to detached medusiform zoöids. On the other hand, many medusid forms produce organisms directly resembling their parents, and many fixed Hydrozoa, such as the Sertularidæ, do not give birth to free-swimming medusoids, but to ciliated gemmules, which, escaping from the capsules in which they had been formed, soon evolve themselves into true polyps. A great part of this "strange eventful history" is still enveloped in darkness, as the life of comparatively but few Hydrozoa has been thoroughly investigated; so much is certain that future observations will bring many new interesting relationships to light, and add new links to the chain which binds together the various members of the hydrozoic class. Although the Ctenophora, thus named from the ciliated bands which constitute so obvious a feature in their physiognomy, closely resemble the Medusæ by their gelatinous consistence and their mode of life, yet a more complex organisation assigns them the highest rank among the Actinozoa, and approximates them to the sea-anemones. The elegant _Pleurobrachia pileus_, which in the summer so often appears on our coasts in countless multitudes, is the species that has been longest known. The melon-shaped body, from half an inch to nearly an inch in length, is clear as crystal, and divided by eight longitudinal equidistant ribs into eight equally large segments or fields. These ribs are covered with numberless flat paddles or ciliæ, placed one above another, and obeying the will of the animal. When it wishes to swim backwards or forwards, it sets all its paddles in motion, whose united power drives the living crystal rapidly and gracefully through the water; and when it wishes to turn, it merely stops their movements on one side. In sunlight, the ribs of the pleurobrachia sparkle with all the colours of the rainbow; in darkness they emit a beautiful cerulean phosphorescence.