60 Colouration in Animals and Plants. 



is their mode of reproduction. Little buds (gonophores) grow from 

 the coenosarc, and gradually assume a form exactly like that of a 

 jelly-fish. These drop off, and swim freely about; and are so like 

 jelly-fishes that they have been classed among them as separate 

 organisms. 



The Tubularise are all transparent ; and in them we find 

 structural colouration finely shown, the colour, as is usual in 

 transparent animals, being applied directly to the different organs. 



Writing of the colour, Prof. Allman says : " That distinct 

 secretions are found among the Hydroida, and that even special 

 structures are set aside for their elaboration, there cannot now be 

 any doubt. 



" One of the most marked of these secretions consists of a 

 coloured granular matter ; which is contained at first in the interior 

 of certain spherical cells, and may afterwards become discharged 

 into the somatic fluid. These cells, as already mentioned, are 

 developed in the endoderm;* in which they are frequently so abun- 

 dant as to form a continuous layer upon the free surface of this 

 membrane. It is in the proper gastric cavity of the hydranth and 

 medusa, in the spadix of the sporosac, and in the bulbous dilatations 

 which generally occur at the bases of the marginal tentacles of the 

 medusae, that they are developed in greatest abundance and 

 perfection ; but they are also found, more or less adundantly, in the 

 walls of probably the whole somatic cavity, if we except that portion 

 of the gastrovascular canals of the medusa which is not included 

 within the bulbous dilatations. 



" In the parts just mentioned as affording the most abundant 

 supply of these cells, they are chiefly borne on the prominent ridges 

 into which the endoderm is thrown in these situations ; when they 

 occur in the intervals between the ridges they are smaller, and less 

 numerous. 



" The granular matter contained in the interior of these cells 

 varies in its colour in different hydroids. In many it presents 

 various shades of brown ; in others it is a reddish-brown, or light 

 pink, or deeper carmine, or vermilion, or orange, or, occasionally, a 

 fine lemon-yellow, as in the hydranth of Coppinia arcta, or even a 

 bright emerald green, as in the bulbous bases of the marginal 

 tentacles of certain medusas. No definite structure can be detected 

 in it ; it is entirely composed of irregular granules, irregular in form, 



* Compare with Hydra above. 



