NERVOUS SYSTEM AND VISION. 93 



to be formed by the almost complete fusion of two ganglions. 

 This ganglion is hardly larger than the chord which it ter- 

 minates : it appeared to me to give rise to more than one 

 pair of nerves, and a single nerve (to my surprise) joined 

 the branch just mentioned, which goes to the adductor 

 scutorum muscle. 



From each supra-cesophageal ganglion, two closely united 

 antennular nerves (/) extend, of which the inner one crosses 

 over the main or exterior nerve, nearly opposite to the 

 ophthalmic ganglion, and here forms (m) a plexus. The 

 structure of this plexus I was not able, any more than in 

 Coronula, to make out thoroughly, but I traced quite dis- 

 tinctly a long nerve (i) running from it into what must be 

 considered as the eye. As in the case of Coronula, I traced 

 a nerve on each side from the ophthalmic ganglion into the 

 plexus, where I lost it ; and as here in Balanus, I saw on 

 each side of the ophthalmic ganglion a cut off nerve, of 

 about the size of that which runs from the plexus on 

 each side into the eye, I think we may safely conclude that 

 the latter or optic nerve does really arise from the ganglion 

 here called ophthalmic. I may add that the analogy of the 

 nervous system in the Lepadidae most strongly confirms 

 the view of this latter being the ophthalmic ganglion. 



The optic nerve (i), running from the plexus to the eye, is 

 of considerable size ; it runs nearly parallel to the main an- 

 tennular nerve, diverging from it a little. It retains nearly 

 the same diameter throughout ; and gives off only one 

 single, small, inner branch. It can be traced beyond the 

 basal edges of the scuta, to just under the upper edge of the 

 transparent opercular membrane, which unites the scuta to 

 the sheath of the rostrum. The nerve itself, at a little dis- 

 tance from its further end, was, in a full-sized specimen, ^ths 

 of an inch in diameter ; widening a little, it expands slightly, 

 and abruptly terminates in a circular disc, about ^ths of 

 an inch in diameter, (see PL 27, fig. 5). The nerve just 

 beneath this slight expansion, is coated all round by pellets of 

 dark purple pigment-cells, but not actually united into a con- 

 tinuous layer. These pigment-cells are the more conspicuous 

 from the surrounding parts being colourless. I could not 

 make out distinctly any cornea ; and I suppose the external 



