ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 729 



is a smaller darker area, due to the outline of the iris, but with a 

 brilliant speck of totally reflected light, due to the lens. Numerous 

 longitudinal canals lodge a specially large stem of soft tissue and 

 nerves, which ramifies towards the surface and terminates either in 

 eyes or in peculiar elongated bodies which are, apparently, organs of 

 touch. From these latter the eyes may be supposed to have arisen by 

 modification. The corneae, which are calcareous, are seen in section 

 to be formed of a series of concentric lamellge ; the pear-shaped cavity 

 of the eye is lined by a dark brown pigmented choroid of a stiff and 

 apparently somewhat chitinous texture. The lens is perfectly trans- 

 parent and strongly biconvex. At some distance from the eye the 

 optic nerve is a com2)act strand, but in the very long tube continuous 

 with the choroid its numerous fine fibres are much separated from one 

 another. The retina is formed on the type of that of Helix ; and not 

 as might be supposed, on that of the dorsal eyes of Oncidium ; it is not 

 perforated by the optic nerve, but it is composed of a single layer of 

 very short, but extremely distinct and well-defined rods, with their 

 ends directed towards the light. A number of the fibres of the nerve 

 do not enter the retina at all, but terminate in small plugs of tissue 

 corresponding to the minor organs of touch ; they appear to form a 

 sensitive zone round each eye. The choroid sacs have a curious open 

 fold which calls to mind the choroid fissure. In some genera — e. g. 

 Chiton — eyes are entirely absent, though the smajl and large touch- 

 organs are present. 



The diflicult problem of the classification of the ChitonidaB will 

 probably be rendered easier, owing to the differences in arrangement 

 and number of the eyes in different genera ; in Corephium aculeatum 

 there must be 3000 on the anterior shell alone, counting only those 

 in good condition ; and on the remaining shells as many as 8500. 



Prof. Moseley has been unable to trace the nerves to their source, 

 but he doubts not that they proceed from the parietal (branchial) 

 nerve. He concludes that the tegmentary part of the shell of the 

 Chitonid^ is something sui generis, entirely unrepresented in other 

 MoUusca. Its chief function seems " to be to act as a secure protection 

 to a most extensive and complicated sensory apparatus, which in the 

 Chitonidse takes the place of the ordinary organs of vision and touch 

 present in other Odontophora," There are some curious resemblances 

 to the Brachiopoda. 



The eyes are ordinarily hard to see on a dried shell with a power- 

 ful lens ; the shell should be wetted with spirit and examined with a 

 lens as powerful as Hartnack's No. 4 objective. 



Eenal Organs of Embryos of Helix* — P. de Meuron describes 

 the primitive renal organs of Helix as arising from ectodermal inva^i- 

 nations, and not as being mesodermal in origin as are, according 

 to Rabl, the kidneys of tho aquatic Pulmonata. The walls of the 

 organ are formed by large cells, with enormous nuclei, which are set 

 in a radiate fashion round the central canal of the tube ; some of 

 the cells become of a particularly large size, as in the forms studied 



* Comptes Rendus, xcviii. (1884) pp. 693-5. 

 Ser. 2.— Vol.. IV. 3 q 



