38 THE VOYAGE OF H.M.S. CHALLENGER. 



V. THE EYE OF LEPAS. 



I believe Leidy was the first who observed in an adult Cirriped an organ of vision.^ 

 In Balanus there are, according to him (and Darwin has confirmed the correctness of 

 his observation), two small eyes which stand apart from each other laterally and, owing 

 to this discovery of the American naturalist, Darvdn^ was led to look for them in 

 Lepadidse. In Lepas fascicularis he found an elongated almost black eye composed 

 of two eyes united together. The eye is innervated by two nerve-chords which extend 

 from the front of the two supracesophageal ganglia, and which before reaching the eye 

 run into two small, perfectly distinct, oval ganglia. From the opposite ends of these 

 two ganglia smaller nerves run, and, bending in at right angles, enter the elongated eye 

 beyond the middle. 



I do not think that any description of this organ has been published since Darwin's. 

 I made preparations of it in Lepas anatifera and in Lepas fascicularis. The place it 

 occupies in the first species may be seen on Plate VI. figs. 7 and 8. On removing 

 the ligament between the two scuta as well as the muscles which are here placed between 

 this ligament and the widened stomach, the surface of the latter with its black (hepatic ?) 

 excrescences and the white pancreatic glands appear. At a distance of about 6 mm. 

 from the supracesophageal ganglion in an adult Lepas anatifera, a small oval black spot 

 is seen attached by means of connective tissue to the surface of the stomach. This is 

 the eye. Morphologically it represents the small pigment spot which, in the Cypris- 

 larva (PI. II. fig. 2, e), is attached to the upper surface of the supracesophageal ganglion, 

 and which is the remainder of the simple eye of the Nauplius-larva. In an adult 

 Lepas anatifera it measures 0"25 mm. only in length, its breadth being not quite 0"15 

 mm. I believe its function to be of no consec^uence, in Lep>as at least, for I do not 

 understand how a ray of light can ever reach it, but the little organ beautifully illustrates 

 the persistence of an old larval structure. 



Most curious, however, is the fact that this rudimentary organ is indeed furnished 

 with a kind of special ganglia (PI. VI. fig. 9). Between the two broad (antennal) 

 peduncular nerves, four thinner ones separate from the supracesophageal ganglion. Their 

 thickness is not quite the same ; the two outer ones are slightly stronger than the other two 

 which lie very close to one another, almost exactly midway between the two other 

 nerves. These four nerves can be traced up to a very short distance (about 0"6 mm.) 

 from the small eye. Here the two stronger nerves of the four bend slightly outwards 

 so as to approach a little more nearly to the peduncular nerves and show a distinct 

 swelling, in the interior of which two elongate ganglionic cells are to be observed. I 



• Leidy, Proc. Acad. Nat. Sci., Philad., No. 1, vol. iv., January 1848. 

 2 Darwin, Lejiadidce, 1851, p. 49. 



