THE EYE OP PECTEN. " 90 



cannot be compared directly with those designations in the 

 vertebrate eye, for the structures bearing these names are not 

 homologous, and in fact the whole structure of the eye is not 

 to be homologised with that of the vertebrate optic organ. 

 The resemblances are pure cases of homoplasy, and there is 

 absolutely no proof of a genetic community of origin. 



Innekvation and General Conclusions. 



It has already been pointed out that the retina is innervated 

 by two branches of an optic nerve which passes down the 

 centre of the eye-stalk (PI. 6, fig. 1 ; PI. 7, fig. 2, Op. N.). 



This nerve has been considered as an offshoot from the 

 circumpallial nerve. In sections which cut the optic nerve 

 obliquely, so that only a small part appears in a section, this 

 may very easily appear to be the case, but if a section cuts 

 the mantle exactly in the plane of the optic nerve, so that a 

 long stretch appears in one section, it will be seen that the 

 real state of things is somewhat different. At irregular 

 intervals nerves pass radially through the mantle- lobes 

 (between the radial pallial muscles) from the visceral ganglion 

 to the circumpallial nerve (PI. 7, fig. 2, Circ. N.). Some of the 

 fibres of these nerves pass into the latter, but at certain places 

 (below the eye-stalks) the bulk of the fibres pass round the 

 circumpallial nerve (on the shell side of it, PI. 7, fig. 2), 

 touching it, but not entering it, and these innervate the eye. 

 Some fibres appear also to leave the circumpallial nerve and 

 to enter this optic nerve, but it will be evident that most of 

 the nerve-fibres come directly from the visceral ganglion. 



Now the visceral ganglion of Pecten is extremely compli- 

 cated in build and I think unique among the Lamellibranchiata. 

 No details will be given here, since a paper is being prepared 

 on this subject, but it will be seen from the figure (text- 

 fig. 1) that there are several lobes, of which two lateral 

 ones are very conspicuous. From these radiate out on either 

 side the pallial nerves {Pall. N.). The ganglion is asym- 

 metrical, the left lateral lobe being larger than the right, and 



