esterly: eucalanus. 35 



Thus far in this paper, I have described the median or tripartite eye 

 of Eucalanus elongatus as a type of this structure among Copepoda. 

 I have tried to show that none of the parts of the eye may be considered 

 as inverted in the sense that the nerves leave the retinal cells from 

 their distal ends. The nerves pass through the basal plates of the 

 optic vesicles and, in part, traverse the entire extent of the "central 

 cell" of the eye, on their way to the brain. It is immaterial, in con- 

 sidering the relation of optic fibres to sensory cells, whether the "cen- 

 tral cell" or the basal plates are held to be the pigment bearers of the 

 eye. The relations of axis cylinders and cells remains the same. 

 There can be no reasonable doubt that there is one nerve fibre, and 

 only one, to each retinal cell, for in several instances such a nerve 

 fibre has been traced from its emergence from a single cell which gives 

 rise to no other fibres; and, furthermore, the number of fibres in the 

 optic nerve and the number of sensory cells in the eye is precisely the 

 same in all cases. It also seems very probable that only the unpaired 

 portion of the eye retains, in the adult, its original relation to the 

 hypodermis; the lateral eyes are without doubt no longer directly 

 continuous with the h^'j^odermis as is the ventral eye. The foregoing 

 are the principal facts which it will be necessary to consider in a general 

 discussion. 



2. The "Inverted" Eyes or "Organs of Claus." 



It remains, now, to describe certain other structures which are proba- 

 bly optical in function. I shall call these "the organs of Claus," from 

 their discoverer. Claus ('63, p. 56) was the first to describe the organs 

 in question, and since that time no one has investigated them in any 

 way, so far as I know. Richard ('91, p. 209) states that they are not 

 found in Cyclops, and Hartog ('88, p. 33) located similar "concretions" 

 at the base of the fifth feet in Cyclops brevicornis. Claus's observa- 

 tions were made upon Eucalanus attenuatus Dana (Calanella medit- 

 terranea); his description of the organs is as follows "Gehororgane 

 wurden nicht mit Sicherheit beobachtet, moglicher Weise aber gehort 

 in die Kategorie dieser Organe eine eigenthlunliche Bildung im 

 Gehirnganglion von Calanella. Es sind zwei kugelige, Gehorblasen 

 ahnliche Riiume, in deren hellem Inhalte ein Ballen von Concre- 

 tionen bemerkt wurde. Ob diese Dift'erenzirung regelmassig auftritt 

 oder nicht, habe ich leider unterlassen zu entscheiden." 



a. Location. — The organs are symmetrically located within the 

 brain at its anterior end (o. Clans, Plate 2, Fig. 24; Plate 3, Figs. 25, 



