62 Colouration in Animals and Plants. 



produced for the sake of decoration, as we suggest, may not owe its 

 direct origin to the process of digestion. The digestive apparatus 

 is the earliest developed in the animal kingdom, and in these 

 creatures is by far the most important; the coelenterata being, in 

 fact, little more than living stomachs. If, then, colouration be 

 structural, what is more likely than that the digestive organs should 

 be the seat of decoration in such transparent creatures ? 



Secondly, as to the distribution of the colour. We find it 

 " frequently forming a continuous layer upon the free surface of " 

 the endoderm, in the " spadix of the sporosac," and in the " bulbous 

 terminations " of the canals, that colour is best developed. In other 

 words, the colour is distributed structurally, and is most strongly 

 marked where the function is most important. 



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 

 pidchella, Fig. 2, PI. 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. Medusae 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 medusa? 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, surround- 

 ing 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."* 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. 



* Allman, op. cit., p. 139. 



