MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 35 
They give off — 
1. Two branches downwards to the propodium. 
2. Two branches upwards to the dorsal parietes. 
3. Two large branches, which run at first separately below the stomach, and then 
unite to form a single trunk. This runs along the stomach and intestine, sometimes 
twisting round them and giving off branches to them and to the parietes ; and on 
the intestine it separates again into two chords, which join two small ganglia (3) 
placed between the aorta and the intestine ; one lies on the aorta, the other be- 
tween this vessel and the intestine, and they are connected by a commissure. From 
the former of these ganglia, which is the smaller, two nerves pass upwards and join a 
flattish mass placed immediately beneath the ‘ subspiral ciliated band.’ There was 
an obscure appearance of branches radiating from this mass, and it is probably 
ganglionic. 
Organs of Sense. The Eyes (fig. 8). — The eyes are very perfectly organized ; each 
eye is contained within a chamber, excavated in a papilla, whose convex wall forms 
a sort of supplementary cornea, answering to the cornea of Cephalopoda, or to the 
corresponding cutaneous cornea of the Gasteropoda. 
Their eye-proper is suspended within this chamber by a number of irregular mus- 
cular bands, which stretch from it to the walls of the chamber, and perform the func- 
tion of ocuU-motores. The optic nerve penetrates the inner wall of the chamber and 
enters the eye from that side. 
The eye-proper is elongated and somewhat hour-glass-shaped, being contracted just 
behind the crystalline lens. The constriction divides the eye into two portions, an 
internal and an external. The latter is almost spherical, and is formed by the true 
cornea, which is much thicker in the middle than elsewhere, so as to present a 
meniscus section. Behind, the cornea is continuous with the sclerotic coat, which is 
thick, and seems to be continuous with the neurilemma of the optic nerve. 
The crystalline lens is spherical, and is separated by a very small interval from the 
cornea, so that there is hardly any anterior chamber of the eye. There is no iris, but 
the inner surface of the posterior chamber is coated by a layer of dark chocolate- 
coloured pigment*. 
The Auditory Vesicles {j)^ fig. 7- — These lie behind and a little below the cephalic 
ganglia. Each is a spherical vesicle, about -^^dth of an inch in diameter. Its wails 
are irregularly thickened here and there, and it contains a spherical otolithe of about 
half its diameter. I was unable to perceive any motion in the otolithe. The auditory 
nerve is a delicate thread arising from the under surface of the supra-oesophageal 
* I do not see that the eyes of Heteropoda are so “ peculiarly formed ” as Kkohn has it (Ferner Beitrag zur 
Kenntniss des Schneckenauges. Muller’s Archiv, 1839). What I have described as the true cornea, is by 
Krohn considered as an anterior portion of the vitreous humour ; such an arrangement would of course be 
very peculiar, but I think that my account is correct. The eye, it would appear, projects much less in Carinaria 
and Pterotrachea ; and in the latter, according to Krohn, there is even a rudimentary eyelid. 
Krohn does not say anything about the muscles of the eye in these two genera. 
