16 
G. H. Parker 
respect, that of cell 7, and thus afford on either side of the reti- 
nular axis a transparent shaft by wbicb light can enter the body of 
the rhabdome directly. It is possible, too, that in the fresh condition 
of the rhabdome the pigmented processes are not so prominent as in 
the preparation figured ; the latter, however, represented a veiy typical 
longitudinal section. 
The rhabdome when taken directly from the live animal has a 
uniformly reddish pink tint, and, as can be seen in longitudinal 
section (Fig. 7), is composed of a series of some twenty-two plates, 
the flat faces of which are transverse to the retinular axis. Fach 
plate is subdivided into two half-plates by a plane vertical to its 
flat face and passing through the retinular axis. The division-planes 
of adjoining plates do not coincide, but are at right angles to each 
other, thus giving rise in each rhabdome to two sets of alternating 
plates ; when the division-planes of one set are cut transversely and 
are consequently seen most clearly (Fig. 7), those of the other set 
are, of course, not visible, since they lie parallel to the plane of 
section. It will be remembered that the sides of the rhabdome 
were designated from their positions as dorsal, ventral, anterior, and 
posterior, and, since the division- planes in the plates are always 
parallel to one or other of these sides, the resulting half-plates may 
be distinguished as dorsal and ventral when the division-plane cuts 
the plate from its anterior to its posterior side, and anterior and 
posterior when the plate is cut dorsoventrally. 
The half-plates of a given side of the rhabdome belong to the 
retinular cell or cells of that side; thus, the ten or eleven posterior 
half-plates belong to the single posterior retinular cell, number 7 
(Fig. 7, la. and represent the rhabdomere of that cell; the corre- 
sponding anterior half-plates belong to the anterior cells, numbers 3 
and 4 (Fig. 7, la. a)^ and represent the united rhabdomeres of these 
cells; i. e., these half-plates, as I shall presently show, are to be 
regarded as fused pairs of quarter-plates, the rhabdomeres in this 
case consisting of quarter-plates instead of half-plates. In the planes 
of the anterior and posterior half-plates, retinular cells 1 and 2, and 
5 and 6 take no part in the formation of the rhabdome, their con- 
tribution to this structure being the ten or eleven dorsal and ventral 
half-plates, which, of course, alternate with the pairs of anterior 
and posterior half-plates and which must also be regarded as com- 
posed of quarter-plates. When the retinula is macerated, as Watase 
(90, pag. 299) has shown in Homarus^ its cells or pairs of cells 
