25 



central ommatidia are plano-convex, with the flat side 

 internal ; those in the peripheral regions have the inner 

 side slightly convex also. 



In a single ommatidium (PI. lY., fig. 1) the following 

 parts can be recognised. The internal face of the corneal 

 cuticular facet {corn, cut.) is covered with two thin cells, 

 the subcoTneal hypodermal cells (.*;.r. hyp.) The nuclei of 

 these cells can be seen in the figure. Internal to these are 

 the nuclei of the two cone cells, [nuc. con.). Each of the 

 cone cells secretes a hemispherical transparent mass (con.), 

 the two segments with tlieir fiat surfaces apposed form the 

 cone. The cone cells surround the cone segments, and on 

 the proximal side form two sub-cylindrical, transparent 

 accessory cones (ace. con.), which is the most interesting 

 and exceptional feature of this eye. The cone cells 

 are surrounded by two pigment cells (jyg. c.) which 

 completely invest the upper half of each ommatidium. 

 The retinula consists of six retinulae cells, and not seven, 

 as stated by Beddard (1888). In this it agrees with 

 Idotea inovata, which also has six retinulae cells (Parker, 

 1891). The retinulae cells [ret.) have fibrillar axes which 

 are continuous with those of the nerve fibres. The six 

 nuclei of the retinulae cells are situated at their proximal 

 ends (nuc. ret.). The rhabdom consists of six individual 

 rhabdomeres, each rhabdomere (rh.) remaining attached to 

 the retinula cell which forms it, and separate throughout 

 its length from the other rhabdomeres. There is a dense 

 mass of pigment (jjg.) between each of the rhabdomeres 

 and its retinula cell. This may have been formed by the 

 retinula cell, which also contains a large amount of 

 pigment, or it may have resulted from an intrusion of a 

 process from one of the pigment cells. The latter view 

 is probably the correct one. The nerve fibrils of the 

 retinulae pierce the basement membrane (h.m.); those 



