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 These animals are very similar to the last, but they are all compound, and the polypites can be entirely withdrawn within the leathery investment or polypary.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 cœnosarc, 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]
b. Physophoridæ. These creatures are distinguished by the presence of a peculiar organ, the float, or pneumatophore, which is a sac enclosing a smaller sac. The float is formed by a reflexion of both the ectoderm and endoderm, and serves to buoy up the animal at the surface of the sea. The best known species is the Physalia, or Portuguese Man-o'-War.