Eyes of Molluscs and Arthropods. 597 



lost, before euteriug the blood Spaces siirroiinding tbe leus, is shown by 

 the fact that. as it bends inwards, it gives off numerous radiating fibres 

 from its out er surface. which enter the peripheiy of the retina, 

 without passing- through the thickeued central part of the septiim. They 

 are the fibres which surround the narrow outer ends of the retinophorae, 

 and of which we bave already spoken. 



The optic nerve and the proximal parts of its two branches are 

 surrounded by a delicate sheath, beneath which is a cortical, nucleated 

 layer surrounding the central fibrous axis. The connective tissue of the 

 stalk contains several large blood sinuses. Tlie numerous radiating 

 arms of the connective tissue cells, at the periphery of the stalk, become 

 more regularly arrauged, and constitute a system of circular and longi- 

 tudinal muscular fibres. The latter, on the branchial and shell side of 

 the stalk, form two, quite well defined groups of large, striated 

 muscular fibres. Toward the base of the eye, are large. scattered, gang- 

 lionic cells, drawn out into several fibres, and filled with granular pro- 

 toplasm (PL 29, fig. 19). 



Besides the tissues already mentioned, there are numerous long and 

 refractive fibres, which arise from the mass of fibres at the periphery 

 of the stalk, and, converging toward the base of the eye, penetrate the 

 sclerotica and the superimposed layers, as far as the inner ends of the 

 rods. In the stalk they are easily seen as single, seldom branched, wavy 

 fibres, which, in the sclerotica, expand into refractive, spindle-shaped 

 bodies, — often of a very faint pink color, — and are then continued stili 

 further inward, either as single fibres, or divided into several branches. 

 These remarkable fibres, concerning whose origin and formation no 

 very satisfactory explanation can be given, — see PI. 32, figs. 151 and 

 152 — are as difficult to preserve, especially the ends with the spindle- 

 shaped swellings, as the two layers of the sclerotica, and, like them, 

 theìr preserva tion depends upon conditions, which I was neither able 

 to understaud nor control. In successful preparations. the number of 

 the spindles seems to vary considerably : in one instance, in which 

 they were preserved with remarkable clearuess, I could only count 

 three or four in each section : in other cases, as many as fifty or sixty 

 could be seen. 



Development of the Eyes of Pecten. 



The foUowing observations concerning the development of the eyes 

 qì Pecten were made upon young specimens, from 1 — 3 mm. long, of 



