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ambulacra! pores into the perivisceral cavity, where they form along the upper side of the 

 skeleton of the arm a double row from the base to the extreme point (see Tab. Ill, fig. 4 

 and 17). The extremely thin-skinned nature of these ampollse in the Brisinga seems to 

 make them well adapted for effecting a transmutation of gas between the water enclosed in 

 them and the fluid of the perivisceral cavity. Moreover the direct reception of water which 

 takes place in the perivisceral cavity itself, chiefly as we may suppose through the terminal 

 section of the ambulacral vessels, must no doubt contribute in an essential degree to the 

 purification and maintenance of the blood. 



e. Secretion. 



I have previously noticed as an apparatus of secretion, the peculiar and strongly 

 developed glandular organ in the Brisinga, which is situated in the' middle above the 

 stomach between the latter and the dorsal skin, and from which there issues a short canal 

 terminating on the dorsal side of the disc in a fine pore with a slightly excentric position. 

 It is this pore which in the Brisinga, and indeed also in other star-fish, has been called 

 the anal aperture, the narrow canal connected with it having been considered as a terminal 

 section of the alimentary cavity or a real intestine. I have already remarked, in describing 

 this apparatus, that in my opinion such a view is erroneous. Between the said canal and 

 the cavity of the stomach, I have not been able to discover any direct connexion whatever; 

 and even if any such connexion really existed, it would still be difficult to understand how 

 this canal could act as a terminal intestine; for it is so extraordinarily narrow, that even 

 the smallest solid particles could not possibly be expelled through it and through the like- 

 wise extremely fine dorsal pore. Neither have I ever seen real excrements expelled in this 

 manner; all the indigestible remains of the aliments are on the contrary ejected again through 

 the mouth. I can not therefore consider this canal in the Brisinga as any thing else than 

 a simple issue for the glandular apparatus previously noticed; and I think it highly probable 

 that many of the star-fishes which have been presumed to possess an anus, are really without 

 it, but have only a similar small secretory pore. 



The product of secretion is a colorless fluid developed in the organ which, ac- 

 cording to its whole structure, is evidently glandulous. Concerning the nature of this secre- 

 tion, it seems reasonable to regard it as a sort of urinary product, on which supposition 

 the whole glandular apparatus would have to be considered as a blood-purifying organ 

 similar to the kidneys in the higher animals. Such a function has also usually been previ- 

 ously ascribed in other star-fishes to the so-called interradial or rectal caeca, to which the 

 organ in question evidently corresponds. 



