68 PATTEN. [Vol. I. 



its pointed inner end (ax.n). The inner portion of the 

 double cell is filled with refractive and colorless globules. (Figs. 

 7 and 8, «^.) 



In the undifferentiated epidermis of the mantle edge of 

 Molluscs the nerves extend along the lateral walls of the 

 cells. (Fig. 1 8, IV. and V.). The fibrillae are applied to 

 the surface of the cells, and usually cling so closely to it 

 that they appear to, and probably do, penetrate the wall of the 

 cell, and stand in direct communication with its protoplasmic 

 contents. The nerve-fibres are therefore inter-cellular. But 

 if two cells, whose lateral walls are well supplied with nerve- 

 fibres, unite, and the apposed walls disappear, those nerve-fibres 

 which were originally between the cells would then lie in the 

 centre of a double cell. The central cells, or retinophorce, of the 

 ommatidia in Lamellibranchiata have been formed in this way, 

 by the fusion of two cells whose apposed walls have disap- 

 peared, allowing the inter-cellular nerve-fibres to form intra- 

 cellular, or axial nerves. In some cases the outer ends of the two 

 cells composing the retinophora have failed to unite ; and, as 

 each end then contains a perfectly normal nucleus, we can clearly 

 see the double nature of the retinophorae. When the union is 

 complete, as in the normal retinophoree, one of the nuclei de- 

 generates and often disappears. The retinophorae are sur- 

 rounded by a circle of pigmented cells, or retinulce, whose inner 

 ends are often reduced to slender hyaline stalks or bacilli (Figs. 

 7 and 8,^^). The retinulae ^ are never double, and therefore 

 never contain an axial nerve-fibre. The cuticulc^, which is often 

 slightly thickened over the pigmented areas containing omma- 

 tidia, usually consists of two layers : a thin and structureless 

 outer one devoid of nerve fibres, the corneal ciiticula (Fig. 6, <r.c.), 

 and an inner, thicker layer, the retinidial cuticula. The latter 

 contains a part of the network of nerve-fibrillae, or retia termi- 



* One meets serious difficulties in attempting to designate the pigmented cells sur- 

 rounding the retinophorae. If we regard the ommatidia as little retinas, then re- 

 tinula-cells would include the retinophorae as well as the pigmented cells. I have 

 used, provisionally, the term retinula; to designate in a general way the pigmented 

 cells surrounding the retinophorae, while in the Arthropods I have used it interchang- 

 ably with the term retinula-cells of Grenacher, in contradistinction to those pig- 

 mented-cells surrounding the calyx. In most cases, I believe, the reader will not be 

 misled. The term, however, as I have used it, cannot be recommended, and it is to 

 be hoped that a better one may be suggested. 



