MOLLUSCS. 769 
In the Cephalopods, the optic nerve is continued into a large 
reniform ganglion, which is surrounded by a white fatty mass 
divided into lobes, and is, with the eye-ball, surrounded by a 
common case, a fibrous membrane, by which a sac is formed, that 
is much larger than the eye-ball, and may be regarded as the cap- 
sule (orbita) of the eye. In front this membrane is attached to the 
common integument. There the skin becomes transparent, and 
sometimes forms, by duplicatures or folds, two eye-lids as it were. 
Behind this transparent membrane, perforated by a round opening, 
the eye-ball is situated. A cornea is not present, and thus also 
there is no anterior chamber, unless the space that intervenes be- 
tween the transparent continuation of the common integument and 
the eye-ball be so named. Within the eye-case lies a tunic of a 
silvery lustre. The eye-ball itself has a cartilaginous external 
membrane, which is perforated behind like a sieve by the filaments 
of the optic nerve, and in front at the margin of the lens forms the 
circle of the pupil. This covering may be regarded as sclerotica. 
Within it lies the expansion of the optic nerve, the retina, which 
also contains a purple-brown pigment-layer. The lens is large, 
elongate, round, and at the posterior surface more convex than in 
front}. X 
The muscles of molluscs are attached in general to the inner 
surface of the skin. They do not exhibit the transverse stripes on 
the primitive bundles which microscopic anatomy has detected in 
the muscles of articulate animals’. Those Gasteropods that have a 
figs. 6—8. Kroun confirmed SwAMMERDAM’S statement of the independent existence 
of lens and vitreous humour; whether an aqueous humour also is present, as SwAM- 
MERDAM concludes, he leaves undetermined. 
1 On the eye of Cephalopods compare amongst others Cuvier Mém. s. les Moll. 
No. 1, pp. 37—41, Pl. 1. fig. 3, Pl. 1. fig. 5; D. W. S@amernine de Oculor. Sectione 
horizontali. Gottinge, 1818, fol. pp. 76—78, Tab. 111.; Kron Now. Act. Acad. Cesar. 
Leop. Carol. N. C. xvii. 1, 1834, pp. 339366, Taf. 26; Ducrs Traité de Physiol. 
comp. Paris, 1838, 8vo, I. pp. 315—318. In Nautilus I am not able, any more than 
OWEN, to discover a trace of the lens; that this part should have escaped through the 
small aperture existing in the middle of the flat anterior surface of the eye, appears to 
me probable. Fresh specimens alone can afford a satisfactory solution. 
? That they occur in Sagitta, is in my judgment a further proof that this animal is 
no mollusc. In Cephalopods also I cannot any more than in the muscular circular 
belts of Salpa (p. 695) discover these transverse stripes, though they have been adopted 
indeed in these animals, and also figured. 
VOL. I. 49 
