Mr. A. Hancock on the Anatomy of Oithona. 297 



manner supplies branches to the various organs. The auricle (c) 

 is united to it behind, a little on the left side ; it is delicate in 

 comparison with the ventricle, but is nevertheless abundantly 

 supplied with muscular fibres ; it lies diagonally in the pericar- 

 dium, having the left side advanced almost to the front of that 

 organ where it receives a trunk-vein from the skin. The right 

 side of the auricle stretches backward, and receives a similar 

 trunk-vein from the skin of this side almost at the posterior ex- 

 tremity of the pericardium. 



On laying the dorsal wall of the auricle open, its cavity is 

 found to be continuous with that of the great posterior elevated 

 median ridge or trunk- vein (d) before alluded to, and on opening 

 this trunk-vein the various lateral branches (f) are observed de- 

 bouching into it on either side. It is therefore evident that this 

 trunk-vein, which lies entirely within the skin, is the great poste- 

 rior efferent or branchio- cardiac vein, and that all the elevated 

 branches coming to it from the papillae are also efferent vessels. 

 In this beautiful system of veins, then, we have a clear proof of 

 the branchial character of the papillae. 



The papillae are, as we are already aware, of a very peculiar 

 character in our animal, being somewhat compressed as in Eolis 

 papillosa j and having a distinct, widish, frilled membrane, extend- 

 ing up their inner margin. It is, as before remarked, to the 

 border of this membrane that the twigs of the efferent vessels 

 are given, and they pass up its entire length. Of this there can 

 be no doubt, for we succeeded in forcing a creamy fluid which 

 pervaded these vessels almost to the top of the membrane. When 

 a transverse section of a papilla is made, a widish canal (fig. 8 b) 

 is seen to pass up the opposite margin. This may be looked 

 upon as an afferent branchial vein, and deteriorated blood, pass- 

 ing from the skin up this canal, may be supposed to filter through 

 the cellular tissue (d) between the external skin and the glandular 

 sheath of the papilla, and so find its way to the vessel (c) at the 

 free border of the membrane. If this view be correct, and it 

 would seem scarcely possible to doubt it, the papillae are evi- 

 dently specialized breathing organs, and by no means so low 

 in organization as has been thought. 



At the same time, from the arrangement of the efferent vessels, 

 from their elevation above the general surface of the skin, thus 

 exposing to the influence of the surrounding medium nearly 

 three-fourths of their circumference, it is pretty clear that the 

 dorsal skin itself must act, to some extent, as a gill, especially 

 when we consider further, that the whole of the blood returned 

 to the heart does not pass through the papillae : much of it, no 

 doubt, circulating in the spongy tissue of the skin, passes at once 

 into the efferent vessels ; and, indeed, small orifices for this pur- 



