ESTERLY: EUCALANUS. 15 
the investigations of Parker (91, p. 81) have shown that there are two 
cone cells in each retina, in addition to eight retinular cells and “other 
nuclei which probably represent undifferentiated cells.” 
It is plain that the number of retinal elements is accurately known 
only in Eucalanus elongatus and E. attenuatus, and that in these 
species the number is definite and constant both as regards the paired 
and unpaired portion of the eye. 
The nuclei of the retinal cells are spheroidal or ovoidal and contain 
a chromatin network. The nuclei of other cells which appear around 
the eye (such as muscle cells, the hypodermis cells, and sheath cells) 
may be distinguished at a glance from those of the retinal cells by 
the difference in staining reaction. ‘This of course appears to best 
advantage in haematoxylin stains, but is also evident in Mallory’s 
connective-tissue stain. Figure 16 (Plate 2) shows the differences 
accurately, and Figure 23 (Plate 2) more diagrammatically. It will be 
seen that the nuclei of the retinal cells invariably contain more chro- 
matin and so stain more deeply. This is an important character, for 
otherwise the nuclei of muscle or connective-tissue cells would be con- 
fused with them. It may be noted, also, that aside from their location, 
the nuclei of the optic cells are indistinguishable from those of the cells 
in the brain. 
e. Interior bodies (Phaosomes); their Arrangement in the Cells— 
The nuclei, however, are not the most noticeable bodies within the 
retinal cells. This characteristic belongs to the “interior bodies” 
(Binnenkérper), which Hesse (:01, p. 350) was the first to describe. 
He found that the interior bodies lie between the nucleus and the 
particular structure (Stiftchensaum) which, in his opinion represents 
the ending of the retinal nerves, but nearer the nucleus. ‘These bodies 
have a greater affinity for stains, as Hesse has stated, than does the 
cell plasma. Hesse considers that they are essentially ribbon-lke 
bodies, which may be so sharply bent or twisted that they seem to be 
divided into separate pieces. He thinks that branching of the bodies, 
if it occurs at all, is rare. He failed to find these bodies in Eucalanus 
attenuatus, but they are, he states, of constant occurrence in Calanus 
gracilis. He is unwilling to commit himself as to the part the interior 
bodies play in the reception of light, but states that they are not neces- 
sary for that function because they are absent from the retinal cells 
of Eucalanus attenuatus. 
The observations ‘of Hesse upon the median eye were made pri- 
marily to determine the method of nerve termination in the retinal 
cells, and he has given scarcely any attention to the interior bodies. 
