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 pignient-layer. The lens is large, 

 elongate, round, and at the posterior surface more convex than in 

 front 1 . 



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 2 . Those Gasteropods that have a 



figs. 6 8. KROHN 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 Mem. s. les Moll. 

 No. i, pp. 3741, PI. i. fig. 3, PI. n. fig. 5; D. W. SCEMMERING de Ocidor. Sectione 

 Jwrizontali. Gottingse, 1818, fol. pp. 7678, Tab. in. ; KROHN Nov. Act. Acad. Ccuar. 

 Leop. Carol. N. C. xvn. i, 1834, pp. 339366, Taf. 26; DuGfes Traitt 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. 



8 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. 



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