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 eve of the calanoid 

 Copepoda can not from its sirnciure 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 eve. 



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 phaosomcs. These have generally a flattened rod-like 

 form and are arranged in such a wav that when sectioned the lono- 

 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 ])lates 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. The 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 irreg;u- 



