20 M. Robin on a peculiar Organ found in the Rays. 



I have already said that its anterior portion was completely 

 surrounded by some concentric muscular layers^ and then that 

 it became subcutaneous in its three posterior quarters, for the 

 greatest part of its surface. I add, by way of more detail, that its 

 internal surface alone is not subcutaneous, and is separated from 

 the vertebral column by the two long muscular and tendinous 

 bundles intended to move the caudal vertebrse. Its upper mar- 

 gin is traversed by a large subcutaneous vein, a branch of the 

 lateral vessel ; its external surface is traversed by the lateral vessel 

 itself, which is accompanied by the lateral nerve. This nerve is 

 situated betweeen the organ and the skin, throughout the whole 

 length of the subcutaneous portion. 



After these details on the relations of this apparatus with the 

 adjacent organs, — relations, moreover, common to all the species 

 of Rays, — I proceed to make known the texture of the tissue 

 peculiar to this organ and the distribution of its vessels and its 

 nerves. For this purpose I shall adopt a comparative course, 

 that is to say, at each step I shall refer to the relations of this 

 organ with those which most resemble it in other fishes. 



On examining attentively the apparatus in question, we ob- 

 serve that its substance does not constitute an uniform gelatinous 

 mass, but that it is divided into a large number of polygonal 

 flattened discs by the partitions of cellular tissue. These discs 

 have consequently two surfaces larger than the rest, one turned 

 forwards, the other backwards. With respect to the faces of the 

 circumference, they are in number three, four or five, which gives 

 the discs a triangular, tetragonal or pentagonal form ; their small- 

 est diameter measures the thickness of the disc, which is 1 mil- 

 limetre in nearly all the species. The diameter of the large sur- 

 faces, which measures the height of the disc, is 2 mill., one more 

 in the Raia rubus and jR. batis, and 3 to 4 mill, in the Raia cla- 

 vata. 



From this difference it follows, in the greatest diameter of the 

 discs according to the species, that in a transverse section of 

 the organ, from eleven to fifteen discs are counted in a R. clavata, 

 and twenty to twenty-five in the R. rubus and the R. batis. The 

 volume of the discs increases with age and the size of the indivi- 

 duals, but their number does not appear to increase. 



These gelatinous discs are piled up one upon another, in the 

 direction of the length of the apparatus, by the adherence of 

 their broad faces, with the interposition nevertheless of a thin 

 cellular partition. These longitudinal rows of discs are arranged 

 side by side, reunited by a cellular partition thicker than that 

 which separates each disc from that which follows or precedes it. 

 The kinds of longitudinal columns represented by the piled-up 

 discs are not rectilinear and do not all follow the length of one 



