ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 223 



puscle is formed by the retina and the outspread optic nerve, while it 

 is closed anteriorly by the inner cell-layer of the pellucida. Within 

 there is either a lens or vitreous body or both ; the eye is completely 

 invested by the connective tissue of the tentacle or ommatophor. 



The retina is formed of a number of similarly constituted groups of 

 cells ; each rod-cell is surrounded by a number of pigment-cells ; as 

 the former are thicker behind and the latter thinner, it follows that the 

 pigment-cell of one rod-cell must pass between the neighbouring rod- 

 cells. The rods have the form of many-sided prisms the anterior end 

 of which, or that which is turned towards the lens or the vitreous body, 

 is slightly convex. In the centre of each rod is an axial portion, but 

 the author was not able to make out the central canal described by 

 Hensen. The axis of the rod appears to be the direct continuation of 

 the anterior part of the rod-cell. 



The lens is always completely structureless in the adult, and no 

 indications of any concentric layers were to be made out ; in the 

 embryo, on the other hand, there are both concentric and radial 

 striations. Its substance is somewhat firm and elastic, and it becomes 

 rather hard after treatment with reagents. The vitreous body is 

 completely homogeneous and transparent. Although Hilger could 

 find no refractive apparatus in the eye of cyclobranchs, he doubts 

 whether it does not really there exist, and supposes that it is lost on 

 treatment ; a supposition which is reasonable enough, when one 

 recollects the open cup-shaped form of the eye. 



In a note to the above paper O. Biitschli * refers to the lately 

 published essay by Grenacher f on the retina of Cephalopods, and 

 points out the important resemblance in structure : both contain two 

 kinds of cells, one with pigment, the other — Hilger's rod-cells — with- 

 out pigment. He takes the opportunity of expressing the difficulty 

 which he feels in accepting the doctrine that the so-called compound 

 eyes of Arthropods owe their origin to the fusion of a number of small 

 simple eyes ; he inclines to the view of Lankester and Bourne that 

 compound eyes have arisen by the differentiation of a common retina, 

 and thinks that the distinct grouping of the retinal elements in the 

 eye of the Gastropoda affords a support to this view. The eyes of 

 Phyllopods have compound retinae and undivided cornese, and though 

 Copepods have a single retinula, it is to be remembered that they are 

 degenerated forms, whereas the Phyllopoda belong to the oldest group 

 of Arthropods. 



Epipodium of Gastropoda.+ — H. de Lacaze-Duthiers, referring to 

 the criticisms of his account of the nervous system which have been 

 somewhat recently made by Dr. Spengel, states that M. Wegmann 

 has, under his direction, gone afresh into the subject, and confirmed 

 the accuracy of his descriptions. After dealing with the question at 

 some length he concludes by reminding his readers that, often as 

 malacologists have stated that the auditory vesicles are connected 



* Tom. cit., pp. 372-5. 



f See this joiirnal, ante, p. 41. 



% Comptes Kendus, c. (1885) pp. 320-5. 



