LIGIA. 89 
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. IV., 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 subcorneal hypodermal cells (s.c. hyp.) The nuclei of 
these cells can be seen in the figure. Internal to these are 
the nuclei of the two cone cells. (nue. con.). Each of the 
cone cells secretes a hemispherical transparent mass (con.), 
the two segments with their flat surfaces apposed form the 
cone. ‘The cone cells surround the cone segments, and on 
the proximal side form two sub-cylindrical, transparent 
accessory cones (acc. con.), which is the most interesting 
and exceptional feature of this eye. The cone cells 
are surrounded by two pigment cells (pg.¢.) 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 (reé.) 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 (nue. 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 (pg.) 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 (b.m.); those 
