_Coelenterata._ These animals fall into two groups, the _Hydrozoa_, of which the hydra and jelly-fishes are types, and the _Actinozoa_, of which the sea-anemonies and corals are types. Most of the coelenterata are transparent animals, but it is amongst them we first come across opaque colouring. Of the lowest forms, the hydras, nothing need be said here, as they are so much like the protozoa in their simplicity of structure. The _Corynida_, familiar to many of our sea-side visitors by their horny brown tubes (_Tubularia_), attached to shells and stones, are next in point of complexity. Within the tube is found a semi-fluid mass of protoplasm, giving rise at the orifice to the polypite, which possesses a double series of tentacles. These important organs are generally of a vivid red colour, thus emphasizing their importance in the strongest manner. Other members of the order are white, with pink stripes. In the larval stage many of the animals belonging to the above and allied orders, are very like the true jelly-fishes. These free swimming larvæ, or _gonophores_, possess four radiating canals, passing from the digestive sac to the margins of the bell, and these are often the seat of colour. In these creatures, too, we find the earliest trace of sense organs, and consequently, the first highly differentiated organs, and they appear as richly coloured spots on the margins of the bell. The true oceanic Hydrozoa again afford us fine examples of structural colouration. The beautiful translucent blue-purple _Velella_, which is sometimes driven on to our shores, is a case in point; and its delicate structure lines are all emphasized in deeper hues. The true jelly-fishes (_Medusidæ_) with their crystal bells and radiating canals, frequently show brilliant colour, and it is applied to the canals, and also to the rudimentary eye-specks, which are frequently richly tinted, and in all cases strongly marked. In the so-called "hidden-eyed" Medusæ we find the same arrangement of colour, the same emphasized eye-specks, and the reproductive organs generally appear as a vivid coloured cross, showing through the translucent bell. Turning now to the _Actinozoa_, of which the sea anemonies and corals are types, we are brought first into contact with general decorative, more or less opaque colour, applied to the surface of the animal. In the preceding cases the animals have been almost universally transparent or translucent, and the colouration is often applied to the internal organs, and shows through. In the sea-anemonies we find a nearer approach to opacity, in the dense muscular body, though even this is often translucent, and the tentacles generally so, often looking like clouded chalcedony. The wealth of colour to be found in these animals gives us a very important opportunity of studying decoration, where it first appears in profusion.