Eyes of Molluscs and Artliropods. 753 



Fig. 102. A fliagrammatic representation of an iniaginary primary and sccondary 

 piate of tlie pedicel, by a nioditication of whicli, the actual platcs, as tliey 

 exist in Penaeus, could bave arisen; the dotted line, «, represents the 

 change necessary to transforui the Hat plates into ones like those seen in 

 figs. 103 and 104. 

 Fig. 103. Adiagram of a primary piate; the fìbres extend from a. toh.; at a. should 

 be an indentation to receive a fold of the pigment cell, 1. (compare 

 figs. 72 and 93). 

 Fig. 104. A diagram of a secondary piate ; the fibres extend from c. to d., therefore 

 at right angles to thoseof fig.103; each piate is divided into two parts by 

 four planes which form an angle , the apex of which lies in the middle of 

 each side of the piate, but is directed in one way on one side, and in the 

 opposite direction lipon the other ; each piece (not strictly a half) then 

 consists of four sraaller and equivalent , but not identical pieces , for the 

 two upper ones are Images of the two lower, while the two right band 

 pieces are bevelled on one side, and the two left band pieces on the oppo- 

 site. Therefore of the eight pieces composing such a piate only two of any 

 ot the four kinds are identical. 



Fig. 1 05. Diagram of two primary and two secondary plates of the pedicel o^Penaeus. 



Fig. 106. Diagram of the basal membrane oiPenaens, seen from the upper surface. 

 Each cross is a thickening in the membrane, and lies directly beneath an 

 ommatidium ; the axial nerve of each ommatidium penetrates the centre 

 ot each cross. Each cross, and therefore each ommatidium, has a capital 

 letter while the ends of the seven retinulae are marked with small letters 

 similar to the large letter of the cross to which they belong. At E. and 

 H. the unseparated retinulae are projected upon the cross to which they 

 belong (compare figs. 97 — 100); in all other cases the retinulae bave taken 

 their respective positions in the corners of the Square Spaces or holes 

 separating the crosses. 



Fig. 107. Four crosses of the basal membrane ofPe??öet<s, seen from the under side; 

 to the centre of each cross are attached several connective tissue fibres c.t.c. 

 In one corner of each cross is a shallow pit, into thebottom of which opens 

 the canal for the axial nerve, c.ax.f. 



Fig. los. Diagrammatic representation of the retinulae, the bacilli, and the nerve 

 fibres, of each ommatidium; the continuation of the cells below the basal 

 membrane represents the bündle of nerve fibres which goes to the base 

 of each cell. Thedrawing represents an imaginary section of the diagram 

 fig. 106, through the points x. and tj. The cells and their nerve fibres are 

 lettered to correspond with fig. 106. 



Fig. 109. A diagrammatic section of the nerve bündle going to the openingsll. and 

 III. of the basal membrane represented in fig. 106. (Compare the letters 

 of these two nerve bundles with those of figs. 108 and 106, II. and III.) 



Fig. 110. Highly magnified and partly diagrammatic view of a basal membrane cross 

 with the ends of the retinulae and bacilli upon it. 



Fig. 111. Inner end of an isolated bacillus with the surrounding nerve fibres. 



Fig. 112. Longitudinal section through the tentacle-tip of Cardium edule. 



Fig. 113. Less highly magnified tentacle of C. edule, showing the pigmented cap at 

 the tip, and the lateral fold x. 



Fig. 1 14. Corneal facets of Galathea, to one of which are attached the corneal cells 

 which constitute a kind of iris ; the centrally situated nuclei are surround- 



Mittheilungen a. d. Zoolog. Station zu Neapel. Ed. VI. 50 



