Prof. Allman gives no hint that the colour may be purely decorative, and is naturally perplexed at the display of hues in such vigour; but if this be one of the results of the differentiation of parts, of the specialization of function, then we can, at least, understand why we find such brilliant colour in these creatures, and why it is so distributed. As an illustration of the _Tubularia_ we have selected _Syncoryne pulchella_, Fig. 2, Pl. VI., and its medusa, Fig. 1. The endoderm of the spadix of the hydranths is of a rich orange colour, which becomes paler as it descends towards the less highly organized stem. Medusæ are seen in various stages of development, and one, mature and free, is shown. In these the manubrium, and the bulbous terminations of the canals are also seen to be coloured orange. In these medusæ we find the first appearance of sensory organs. They consist of pigment-cells enclosed in the ectoderm, or outside covering; and are singular as presenting the first true examples of opaque colouring in the animal kingdom. They are associated with nerve cells attached to a ring of filamentous nerve matter, surrounding the base of the bell. In some important respects the pigment differs from that in other parts of the animal. It is more definite in structure; and the whole ocellus is "aggregation of very minute cells, each filled with a homogeneous coloured matter."[25] These ocelli, and similar sense organs, called _lithocysts_, are always situated over the bulbous termination of the canals. The pigment is black (as in this case), vermilion, or deep carmine. [Illustration: Plate VI. SYNCORYNE PULCHELLA.] The dependence of colour upon structure is thus shown to hold good throughout these animals in a most remarkable manner, and the acceptance of the views here set forth gives us an insight into the reasons for this colouration which, as we have seen, did not arise from the study of the question from the ordinary point of view. _C. Sertularida._ These animals are very similar to the last, but they are all compound, and the polypites can be entirely withdrawn within the leathery investment or polypary. Their mode of reproduction is also similar, and their colouration follows the same general plan. Being so like the preceding order, it is unnecessary to describe them. _B. Siphonophora._ The Siphonophora are all free-swimming, and are frequently called Oceanic Hydrozoa. They are divided into three orders, viz.:-- _a. Calycophoridæ._ _b. Physophoridæ._ _c. Medusidæ._ _a. Calycophoridæ._ These animals have a thread-like coenosarc, or common protoplasm, which is unbranched, cylindrical, and contractile. They are mostly quite transparent, but where colour exists it is always placed structurally. Thus, in _Diphyes_ the sacculi of the tentacles are reddish, in _Sphæronectes_ they are deep red, and in _Abyla_ the edges of the larger specimens are deep blue.[26]