Details of Ccelenterata. 61 



and usually aggregated into irregularly shaped masses in the interior 

 of the cells. It is to this matter that the colours of the Hydroida, 

 varying, as they do, in different species, are almost entirely due. 



" The coloured granular matter is undoubtedly a product of true 

 secretion ; and the cells in which it is found must be regarded as true 

 secreting cells. These cells are themselves frequently to be seen as 

 secondary cells in the interior of parent cells, from which they 

 escape by rupture, and then, falling into the somatic fluid, are carried 

 along by its currents, until, ultimately, by their o\vn ruptuie, they 

 discharge into it their contents. 



"We have no facts which enable us to form a decided opinion as 

 to the purpose served by this secretion. Its being always more or 

 less deeply coloured, and the fact of its being abundantly produced 

 in the digestive cavity, might suggest that it represented the biliary 

 secretion of higher animals. This may be its true nature, but as yet 

 we can assert nothing approaching to certainty on the subject; 

 indeed, considering how widely the cells destined for the secretion 

 of coloured granules are distributed over the walls of the somatic 

 cavity, it would seem not improbable that the import of the coloured 

 matter may be different in different situations ; that while some of it 

 may be a product destined for some further use in the hydroid, more 

 of it may be simply excretive, taking no further part in the vital 

 phenomena, and intended solely for elimination from the system."* 



Here we have very definite statements by a highly trained 

 observer of the distribution of colour iii the whole of these animals, 

 and of the conclusions he draws from them. 



Firstly as to the colour itself. We find it true colour brown, 

 pink, carmine, vermilion, orange, lemon-yellow, and even emerald 

 green ; a set of hues as vivid as any to be found in the animal 

 kingdom. It is difficult to conceive these granules to be merely 

 excrementitious matter ; for in such simple creatures, feeding upon 

 such similar bodies, one would hardly expect the excretive matter to 

 be so diversified in tint. Moreover, excrementitious matter is not, 

 as a rule, highly coloured, but brown. Thus, we see in the 

 Rhizopods the green vegetable matter which has been taken in as 

 food becomes brown as the process of assimilation goes on ; and, 

 indeed, colour seems almost always to be destroyed by the act of 

 digestion. 



Still, it by no means follows that this colour, even if it is 



* Allman. Monograph of Tubularian Hydroida. Bay. Soc., p. 135. 



