SECRETION. 135 



to he seen as secondary cells in the interior of parent cells, from which they escape by rupture, 

 and then falling into the somatic fluid, arc carried along by its currents, until ultimately, by their 

 own rupture, they discharge into it their contents. 



We have no facts which may 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 impro- 

 bable that the import of the coloured matter may be different in different situations ; that while 

 some of it may be a product destined for further use in the economy of 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. 



The fluid which acts so powerfully as a solvent on the food which has passed into the 

 digestive cavity, must certainly be also regarded as a secretion. We are ignorant of its exact 

 source, but it is in all probability derived, like the coloured matter, from cells developed in the 

 walls of the cavity.^ 



Under the head of specific secretion must also probably be classed the fluid contents of the 

 thread-cells. 



That the remarkable green matter which is contained in the endodermal cells of Hydra 

 viridis is a special product of these cells there can be no doubt. Its very definite structure, how- 

 ever, would seem to take it out of the class of ordinary secretions, and place it rather in the same 

 group of products as spermatozoa and thread-cells. 



The researches of Cohn' have led him to believe that the green colouring matter of Hydra is 

 identical with that of Euc/lena, and with that of Loxodes, Stenfor, and certain other green Infusoria; 

 and, further, that in all these cases it is indistinguishable from the chlorophylle granules of certain 

 Alija, especially of Faticheria. He has shown that the green granules in these animals, as well as 

 in a green Flanaria which he has also examined, present precisely the same appearances under 

 the action of sulphuric acid as those which we witness in the chlorophylle of plants when sub- 

 jected to the same treatment ; for instead of remaining unchanged, or merely becoming charred, 

 the granules, when brought into contact with the acid, pass in a very characteristic way fi-om 

 verdigris-green to a more intense bluish-green, and at last, in solution, become almost blue. 



From these facts Cohn concludes that the green matter in the organisms mentioned, at least 

 in Eughna and the green ciliate Infusoria, performs a function similar to that of the chloro- 

 phylle of plants, and he regards it as destined for the excretion of oxygen. 



The chitinous perisarc which, to a greater or less extent, invests the surface of almost 

 every hydroid, is perhaps rather a product of metamorphosis of tissue than of true secretion, the 

 most external portions of the ectoderm becoming converted into the perisarc, which increases 

 in thickness by successive additions to the inner surface, these additions giving a distinctly 



' The cells described by Haeckel as arranged in peculiar leaf-like groups in the stomach-walls for 

 certain medusaj belonging to the family of the Geryonidai, -srould seem to be destined for the elaboration 

 of some special secretion. 



- Cohn, "Beitr. zur Entwckel. der Infusorien," ' Zeit. f. Wiss. Zool.,' vol. iii, 1<Sj1. 



