In _Balanus tintinnabulum_, the structure of the great infra-œsophageal ganglion (Pl. 27, fig. 2, A) is essentially the same as described under Coronula. The great pair of splanchnic nerves springing from its under side, are here actually twice as large as the circa-œsophageal chord. The plexus (_d′_) formed by the splanchnic nerve (_d_), on each side, with the supra-splanchnic nerve (_e_), which arises close posteriorly to the supra-œsophageal ganglion, is here much less complicated, but is perfectly distinct; and there was no appearance of the cervical ganglion of M. Martin St. Ange. The chord (_g_) running from between the two supra-œsophageal ganglions to the ophthalmic ganglion, is nearly as large as the double antennular nerve (_f_) on each side of it. The ophthalmic chord (which is accompanied in its whole course by a small branch running to the adductor scutorum muscle) terminates in a small ophthalmic ganglion (C), which seems to be formed by the almost complete fusion of two ganglions. This ganglion is hardly larger than the chord which it terminates: 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-œsophageal ganglion, two closely united antennular nerves (_f_) 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 distinctly 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 Lepadidæ most strongly confirms the view of this latter being the ophthalmic ganglion. _Eyes and Vision._