50 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
same time, he justly states that the relationships of various groups 
can be determined neither by the structure of one organ nor of one 
system of organs; the entire organization must be considered. 
With these considerations in mind, it may fairly be concluded that 
the occurrence of inverted “ Pigmentbecherocellen”’ among Copepoda, 
points strongly toward the relationship of this group with the polychaete 
annelids, particularly if the entire structure of the organisms is taken 
into consideration. On the other hand, the median eye of the calanoid 
Copepoda can not from its structure be regarded as ‘‘eine Erbschaft 
von wahrscheineich plathelminthenartigen Vorfahren,” even though it 
may represent the persistent nauplius eye. 
IV. Summary. 
1. The median eye of Eucalanus is of the well-known tripartite 
type. Each lateral ocellus consists of two basal plates, and of nine 
retinal cells. ‘The ventral ocellus of the eye contains ten cells, and is 
provided with a single basal plate similar to those of the lateral por- 
tions of the eye. 
2. The basal plates are products of the retinal cells, and probably 
do not contain the pigment of the eye. ‘This is believed to lie in a 
central cell, upon or in which the three divisions of the eye rest. ‘The 
tapetum lies upon the peripheral margins of the central cell. 
3. ‘The retinal cells are provided, in their cytoplasm, with “interior 
bodies” or phaosomes. ‘These have generally a flattened rod-hke 
form and are arranged in such a way that when sectioned the long 
axis of their section corresponds with the long axis of the section of the 
cell, whatever the plane of section may be. 
4. ‘The axis cylinders of the optic nerves leave the retinal cells at 
the basal or deep ends (those adjoining the pigment cell), and pass 
through, or to one side of, the basal plates to enter the central cell. 
The individual fibres traverse the central cell toward the brain. 
5. There are twenty-eight fibres in the optic nerve. ‘The number 
corresponds exactly with the total number of retinal cells in the entire 
eye, and it is therefore highly probable that one fibre comes from each 
cell. he fibres may be traced in their individuality some distance 
into the brain. 
6. The terminations of the nerves in the sensory cells are not in 
the form of a ‘‘Stiftchensaum.” The neurofibrillae are rather irregu- 
