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



within. In a word, this organ, separated from the tissues to which 

 it adheres, presents the form of an elongated fusiform body, 

 swollen in the centre, more or less blunt at the extremity, and 

 always flattened upon its internal surface. 



In a Ray one metre* long, the tail was 49, and the organ 36 

 centimetres in length ; 1 1 millimetres in the transversal du*ection, 

 and 13 from above downwards. 



The tissue of this organ has the semitransparency of gelatine, 

 but more consistence, and its colom* is a clear translucid gray. 

 It is furnished with a general fibrous envelope, which adheres to 

 the adjoining tissues by aponeurotic membranous layers ranged 

 at regular distances. 



I have already said that its anterior poi-tion was completely 

 surrounded by some concentric nmscular 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 sm'face alone is not subcutaneous, and is separated from 

 the vertebral column by the two long muscular and tendinous 

 bundles intended to move the caudal vertebrae. 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. 



i\iter 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 

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

 nerves. For this pm-pose 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. ^A'ith 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 R. batis, and 3 to 4 mill, in the Raia cla- 

 vata. 



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

 * The metre is equivalent to 39-37 English inches. 



1 



