Like most of the deep-sea Hydroids, the Pectyllidæ are usually devoid of sense organs, but a single specimen of _Periphylla mirabilis_, captured by the naturalists of the ‘Challenger,’ possessed well-marked eyes. There is also a peculiar family of the Siphonophora, called the Auronectæ, consisting of a few specimens that have been hitherto found only in very deep water. Like the well-known Portuguese man-of-war _Physalia_ of the surface waters, the Auronectæ possess a large swimming bladder or pneumatophore, but they have in addition another peculiar bladder-like cavity, called the aurophore, communicating with it, which may be an organ for secreting gas. A very interesting genus allied to Velella was also found in depths of over 2,000 fathoms by the ‘Challenger’ expedition. It is supposed to be a survival of the ancestral form of the Disconectæ, or, at any rate, to be a link connecting the Siphonophora with the Medusæ. The very well marked octoradial arrangement of the parts of _Discalia_, as this genus has been termed, is certainly a point of great interest and importance. There is no large family of the sea anemones that is peculiar to deep water, but several genera that occur only in the abyss exhibit some curious modifications. The manner in which the tentacular pores have become enlarged, and the tentacles themselves diminished in size and flexibility, has already been referred to in a previous chapter (p. 36). The family of sea anemones that has been named the Corallimorphidæ, characterised by the stiffness and slight contractility of the body, the knobbed nature of the tentacles, and their distribution in several series, was, until quite recently, considered to be a true abysmal family. The two species, _Corallimorphus rigidus_ and _C. profundus_, are known to occur only in very deep waters, and present some curious modifications of structure in relation to their habit; but it seems probable that to this family should be added the remarkable littoral form _Thelaceros rhizophoræ_ found on the coast of Celebes attached to the roots of the mangrove trees in the swamps. The fact that all the principal groups of the Actiniaria, except, perhaps, the group that includes those forms with only eight mesenteries, the Edwardsiæ, have representative genera or species in the great depths of the ocean, points to the conclusion that the sea anemones have migrated from the shallow waters in comparatively recent times, and that the migrations have been successive, each period of their history sending some specimens to survive or to become extinct in the struggle for life in the deep sea. Of the Madreporarian corals, several genera are now known to inhabit very deep water, but they do not present many very remarkable points of divergence from the shallow-water forms.