106 MICROSCOPICAL STUDIES. 



Fig. B. Young of L. Forbes it just after it has broken through the 



egg membrane ; chromatophores shown relaxed. 

 Fig. C. The same viewed as a transparent object. /. fins ; g. gills ; 



z. ink sac ; rd. radula ; s. siphon ; x, chromatophores ; 



y. remnant of yolk sac, as yet unabsorbed ; the eyes are 



shown in optical section. 



(All these figures are from original sketches). 



STUDY XXIII. THE VISUAL ORGANS OF THE MOLLUSCA. 



The organs of sight in their frequently parfect adaptation to the 

 sensory function they perform, claim our admiration in a degree 

 greatly superior to that which we acoord to any other organ of the 

 body. Nor is our interest lessened when we see the manifold modi- 

 fications of structure presented by them ; how, even in the same 

 phylum, one family may be endowed with such organs formed in the 

 most complex manner, whilst closely related forms may possess but 

 the simplest of structures, crudely functioning towards a similar 

 end ; how here, the optic mechanism is prominently and closely 

 associated with the brain ; how, among others, organs arise inde- 

 pendently from the most diverse regions to assume a physiological 

 equality with the cephalic eyes of other types. 



Nowhere do we find greater variability in the origin and structure 

 of the visual organs than among the Mollusca. Thus, while the 

 Gastropoda and the Cephalopoda possess paired cephalic eyes, the 

 Lamellibranchs possess none, their place being taken by numerous 

 peripheral eyes arranged along the edge of the mantle. 



In structure, apart from the mere pigment spots that are borne 

 upon the extremities of the siphons in Solen, Venus, &c., there exist 

 five principal types of eye among the Mollusca. 



The simplest form, found in the eyes of the Limpet (Patella), is 

 a mere bowl-shaped depression of the epidermis, the lining of which 

 has become modified into a visual layer, or retina, into which the 

 optic nerve penetrates by numerous fibre-branches. A slightly higher 

 modification is seen in the eyes of Nautilus, where the ocular pit is 

 excavated out of a broad conical stalk, and with the aperture con- 

 stricted to a mere pin-hole, the whole recalling forcibly both the 

 outline and the section of a young Fig-fruit. In this eye, as in that 

 of the Limpet, the retina is bathed directly by the sea- water. (Figs. 

 II. and IX, PL VII.). 



The second type of Molluscan eye shows considerable advance 

 upon the first. It has the form of a closed capsule and is directly 

 derivable from the simple cup-form, by the ingrowth and fusion of 

 the lips of the cup. This optic capsule usually separates from the 

 epidermis from whose ingrowth it arose, the superficial epidermal 

 layer closes over, becomes transparent; and then may be termed a 

 cornea, as it becomes a transparent protective window for the capsule 



