8 bulletin: museum of comparative zoology. 



the cells rather than with the structure of the eye as a whole. Most 

 investigators have based their studies upon the fresh-water forms, such 

 as Cyclops or Diaptomus, while but three, so far as I am aware, have 

 had an opportunity to study the marine forms. 



Claus ('63) mentions no features of the eye of Eucalanus (E. atten- 

 uatus Dana) except its general form, the optic nerve, and the relation 

 of the frontal nerves to the eye. Grenadier ('79), working upon the 

 same form, made a closer study, but employed only isolation prepara- 

 tions or entire ones. His observations were confirmed in some respects 

 by Claus ('91). Hesse (:01) was the first to make sections of the eye 

 of Eucalanus. 



Grenadier ('79, p. G3) has given an accurate account of the optic 

 organ. He found that it conformed to the well-known tripartite type, 

 or "x-shaped pigment spot," consisting of a ventral unpaired portion 

 and two dorsal paired parts, each of the three divisions possessing a 

 special pigment plate. To each of the pigment plates lielongs a very 

 constant number of cells, all of which, though not uniform in shape, 

 turn an attenuated end toward the point of exit of the optic nerve, 

 which lies between the three divisions of the eye. Each of the paired 

 portions of the eye, or the "lateral eyes," contains eight cells arranged 

 in two layers. The upper layer contains five cells and the loAver layer 

 three. The median, ventral or unpaired eye contains ten cells, eight 

 of which are arranged in pairs, two being unpaired. The ojitic nerve 

 fibres* continue into the cells, entering them at their pointed ends. 

 Grenacher states that the fibres could be traced within the cells in 

 some cases. Claus ('91, p. 229) states that Grenacher failed to notice 

 that the nerve fibres enter from the outer side, thus giving the eye the 

 nature of an inverted " Becherauge," but (p. 246) confirms Grenacher's 

 observations as to the number and position of the cells. Hesse (:01, p. 

 350) mentions the eye of Eucalanus very briefly, merely pointing out 

 the character of the nerve endings and the presence of the "interior 

 l)odies" (Binnenkorper). 



I have studied the eye of Eucalanus both by means of sections and 

 from preparations of the entire organ. The general form of the eye 

 is shown in Plate 1, Figure 1, and has already been described by Claus 

 ('63, Taf. 7, Fig. 9) and by Grenacher ('79', Taf. 5, Fig. 36; Taf. 6, 

 Fig. 37, 38) in E. attenuatus. I have been unable to obtain any speci- 

 mens of this species, but, so far as I can judge from these descriptions, 

 the shape and more general features of the eye are the same as in E. 

 elongatus. 



