Details of Coelenterata. 63 



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 they are all compound, and the polypites can be entirely withdrawn 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 repro- 

 duction 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. Calycophoridtv. 



b. Physophoridiv. 

 e. Medusidce. 



a. Calycophoridce. These animals have a thread-like coenosarc, 

 or common protoplasm, which is unbranched, cylindrical, and con- 

 tractile. 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 Sphceronecles they are deep red, and in Abyla 

 the edges of the larger specimens are deep blue."* 



b. Physophoridce. These creatures are distinguished by the 

 presence of a peculiar organ, the float, or pneumatopkore, 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. 



Prof. Huxley, in his monograph on the Oceanic Hydrozoa, gives 

 many details of the colouration ; and, not having had much oppor- 

 tunity of studying them, the following observations are taken from 

 his work. It will be seen that the Physophoridse illustrate the 

 structural distribution of colour in a remarkable manner. 



Stephanomia amphitridis, the hydrophyllia, colourless, and so 



* Huxley. Oceanic Hydrozoa, pp. 32, 46, 50. 



