36 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
ganglion, just in front of the commissural cord. It appears to terminate suddenly on 
entering the vesicle. 
This origin of the auditory nerves from the cephalic ganglia, when the pedal ganglia 
are well-tnarked and placed helow the oesophagus, is a circumstance common to all the 
Heteropoda*, and, so far as I am aware, altogether peculiar to them among the 
Mollusca. The only writers who appear to have been struck by it are MM. Frey 
and Leuckart : they say that the auditory organs are united with the supra-cesopha- 
geal ganglia “ only when the lower oesophageal ganglia are wanting (except in Cari- 
naria, in which the great length of the lateral commissures of the oesophageal ring 
appears to have made such a position necessary).” (Beitrage, p. 55.) I must confess 
I do not see the force of this explanation ; cind the lower oesophageal ganglia are 
never wanting,” though they may be united with the upper ones. 
II. Anatomy of Atlanta. (Plate III.) 
This is a very small and very beautiful pelagic raollusk, with a shell not more than 
one-fourth of an inch in diameter. It appears to be identical with the Atlanta 
Lesueril of Eydoux and Souleyet. Its structure resembles that of Firoldides in all 
its essential points, and the transition between the latter and Atlanta is complete 
through such forms as Firola, Carinaroides and Carinaria. 
The shell is flattened and spiral, none of the whorls projecting beyond the plane 
of the outermost. The aperture is notched on its dorsal edge, and a deep thin crest 
surmounts the outer whorl, and is generally broken in several places. The surface 
of the shell is marked by delicate transverse striations. 
The outer fourth of the outer whorl of the shell is occupied by the mantle, the rest 
of the spire containing the viscera. 
When protruded, the body of the animal is as large as the shell and appears trifid; 
the large head forming the anterior division, the “fin” the middle, and the “tail,” 
with its operculum, the posterior division. 
The head is large and subcylindrical. Its anterior extremity is formed by a 
circular lip surrounding the mouth. The eyes are placed far back, and the longish 
conical tentacles proceed from the anterior part of their base. 
The fin or “ propodium ” {pp) is flattened and fan-shaped ; its edge is provided with 
many long and delicate hairs, and its surface is covered with little asperities. Just 
below its point of attachment, the posterior edge of the propodiura carries a cup- 
shaped disc {ms), also fringed with long hairs. This is commonly called the “ sucker,” 
and has no representative in Firolo'ides. It may be called the mesopodium. 
The “ tail ” or 7 netapodium-f- {mt) is subcylindrical at its base, but becomes flattened 
* Compare Milne-Edwauds, Sur divers Mollusques, Annales dcs Sciences, 1842, and the figures of 
Eydoux and Souleyet, so often referred to. 
t That this is the same organ as the metapodium of Firoldides, will be obvious upon compai'ing the different 
forms which it assumes in Carinaria, Carinaroides and Firola. 
