134 PHYSIOLOGY. 



The currents descriljcd aliove, as existing in tlie somatic cavity of the Hydroida, serve not 

 only for the distribution of the nutritive fluids among the tissues — an office in which they admit 

 of comparison with the somatic circulation of the blood in the more specialised members of the 

 animal kingdom — but they also contribute to liring this fluid into more immediate relation with 

 the surrounding medium, and thus subserve the function of respiration. 



No differentiated respiratory organs occur anywhere among the Hydroida, and the inter- 

 change between the nutritious fluid and the surrounding medium must take place through the 

 general surface of the body, more especially through those portions of it which, not being under 

 cover of the chitinous perisarc, are more directly exposed to the conditions under which such 

 interchange may be effected. 



The cilia which cover the external surface of the hydroid when in the stage of planula, as 

 well as those which clothe the free sporosac of Dicoryiie, and those which occur on certain hydroid 

 medusBe not yet traced to a trophosome, such as Tracltynema, Gegeng., are probably subservient 

 to respiration as well as to locomotion. 



3. Secretion. 



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 endoderra, in 

 which they are frequently so abundant 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 medusa, that they are developed in greatest abundance and perfection ; but they 

 are also to be found more or less abundantly 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 carmhie or vermilion or orange ; or occasionally a fine lemon-yellow, as in the 

 hydranth of Coppiiiia arcta, or even a bright emerald green, as in the bulbous bases of the mar- 

 ginal tentacles of certain medusa;. No definite structure can be detected in it ; it is entirely 

 composed of minute granules, irregular in form, 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 arc themselves frequently 



