VOL. LXV.] PHILOSOPHICAL TBANSACTIONS. 667 



organ : however, the tail part is likewise adapted to the progressive motion of 

 the whole, and to preserve the specific gravity ; for the spine, medulla spinalis, 

 muscles, fin, and air bladder, are continued through its whole length. Besides 

 which parts, there is a membrane passing from the spine to that fin which runs 

 along the belly or lower edge of the animal. This membrane is broad at the 

 end next the head, terminating in a jx)int at the tail. It is a support for the 

 abdominal fin, gives a greater surface of support for the organ, and makes a 

 partition between the organs of the two opposite sides. 



The Organs. — The organs which produce the peculiar effect of this fish, 

 constitute nearly one half of that part of the flesh in which they are placed, 

 and perhaps make more than one-third of the whole animal. There are 2 pair 

 of these organs, a larger, and a smaller ; one being placed on each side. The 

 large pair occupy the whole lower or anterior, and also the lateral part of the 

 body, making the thickness of the fore or lower parts of the animal, and run 

 almost through its whole length ; viz. from the abdomen to near the end of the 

 tail. It is broadest on the sides of the fish at the anterior end, where it 

 incloses more of the lateral parts of the body, becomes narrower towards the 

 end of the tail, occupying less and less of the sides of the animal, till at last it 

 ends almost in a point. These two organs are separated from one another at 

 the upper part, by the muscles of the back, which keep their posterior or upper 

 edges at a considerable distance from one another ; below that, and towards the 

 middle, they are separated by the air bag; and at their lower parts 

 they are separated by the middle partition. They begin forwards, by a 

 pretty regular edge, almost at right angles with the longitudinal axis of the body 

 situated on the lower and lateral parts of the abdomen. Their upper edge is a 

 pretty straight line, with small indentations made by the nerves and blood 

 vessels, which pass round it to the skin. At the anterior end they go as far- 

 towards the back as the middle line of the animal ; but in their approach 

 towards the tail they gradually leave that line, coming nearer to the lower 

 surface of the animal. The general shape of the organ, on an external or side 

 view, is broad at the end next the head of the animal, becoming gradually nar- 

 rower towards the tail, and ending there almost in a point. The other surfaces 

 of the organ are fitted to the shape of the parts with which they come in con- 

 tact , therefore on the upper and inner surface it is hollowed, to receive the 

 muscles of the back. There is also a longitudinal depression on its lower edge, 

 where a substance lies, which divides it from the small organ, and which gives a 

 kind of fixed point for the lateral muscles of the fin. Its most internal surface 

 is a plane adapted to the partition which divides the two organs from one 

 another. The edge next the muscles of the back is very thin, but the organ 

 becomes thicker and thicker towards its middle, where it approaches the centre 



4'q 2 



