184 Messrs. Hancock and Embleton on the Anatomy of Eolis. 
addition to these, there is a pair of ganglia, ee, at the base of the 
dorsal tentacles, which we call olfactory ; and we have seen what 
we take to be other ganglia, but of these we shall speak further on. 
The nervous centres intercommunicate by the following com- 
missures. A short broad one, f, unites the first pair of supra- 
cesophageal, and a similar though smaller, g, the first pair of 
infra-cesophageal ; these have been already noticed; then the 
lateral supra-cesophageal are united to the first or anterior or 
cerebral by a broad flat band, , so short that the ganglia appear 
to be continuous with each other. Next we have three nervous 
bands or collars, concentrically arranged, inclosing the cesopha- 
gus, and serving to complete the connexions of the supra-ceso- 
phageal ganglia with each other, and to bring them into asso- 
ciation with the infra-cesophageal. First, the innermost or 
thickest collar, 2, lies close to the cesophageal wall, and is com- 
posed of four or five distinct nervous filaments running parallel 
to each other, and connecting together the posterior borders of 
the two lateral supra-cesophageal masses. Second, a slender, 
delicate collar, 7, lies next outside, much wider than the former, 
and uniting the posterior and outer parts of the first pair of 
supra-cesophageal ganglia, it comes out from the under surface 
of these bodies and runs under the second or lateral ganglia. 
The existence of this collar or commissure between the posterior 
parts of the median cerebral ganglia, whilst their anterior parts 
are united by the anterior median commissure, seems to confirm 
the impression we received at first sight, that the cerebral ganglia 
are each of them double centres. It will be observed that the 
two last-described cesophageal collars are not attached in any 
way to the infra-cesophageal ganglia. The third or outermost 
collar, k, however establishes a communication between the first 
or median supra-cesophageal and the first infra-eesophageal gan- 
glia.. This is a strong band, being little inferior in size to the 
first, of uniform texture, and lying just outside of the second col- 
lar, and in contact with it, it is the widest of the three. In front 
it is attached to the under part of the outer border of the first 
cerebral ganglia, considerably in advance of the coming off of the 
second collar ; from this part it is traced backwards under the 
lateral supra-cesophageal into the external end of the buccal 
ganglia. 
The nerves vary a good deal in size, and we have been able to 
trace thirty-three pairs; of these, twenty-one come off from the 
supra-cesophageal ganglia, six from the infra-cesophageal, and 
five from the commissures. There is also a large pair which 
comes out from the buccal mass from an obscure ganglion im- 
bedded in the muscular tissue, and a small nerve, apparently 
single, that separates from the middle collar of the cesophagus, 
