410 NATHAN FASTEN. 



further crossed by vertical lines breaking them up into small 

 squares. The surface of each ocellus thus appears to be made 

 up of numerous facets, very similar to the facets of ommatidia. 

 Figs. I and 2 show this appearance. Fig. I, which is a drawing 

 of a ventral view of the eye, shows the facet-like surfaces and 

 the semi-lunar cups particularly well. 



INTERNAL STRUCTURE OF THE EYE. 



The true structure of the eye is revealed when sections of the 

 organ are studied under the microscope. In Fig. 3, which is a 

 transverse section of the larval organism, the tripartite eye (e) 

 is seen to occupy the middle space between the brain (b), and 

 the dorsal wall of the body (w). The details of the eye can best 

 be seen in Figs. 4 and 5. Fig. 4 is an enlarged camera-lucida 

 drawing of the eye seen in Fig. 3, while Fig. 5 is a drawing of a 

 frontal section of the eye. In size, the two lateral ocelli (Fig. 4, 

 /. o) are equal, while the median ocellus (Fig. 4, m. o] measures 

 about two thirds the dimensions of either of the aforementioned 

 ones. Furthermore, as already stated, these ocelli are imbedded 

 in semi-lunar cups (Figs. 4 and 5, c) which touch each other 

 closely. The inner surface of each cup is thickened into a basal 

 plate (Figs. 4 and 5, r) which stains a heavy black with Heiden- 

 hain's iron-haematoxylin. This plate comes in contact with the 

 ocellus and, in all probability, is its most sensitive portion. 

 Esterly ('08) found that in Eucalanus elongatus the lateral ocelli 

 possessed two basal plates, while the median ocellus contained 

 only one. In Salmincola edwardsii this difference was not ob- 

 served. Here each ocellus bears a single plate. Between the 

 open spaces of the semi-lunar cups the pigment granules of the 

 eye are found distributed (Fig. 4, p). 



Upon closer examination each ocellus is observed to consist 

 of a definite number of cells, the so-called retinal cells (see Figs. 

 4 and 5), there being nine in either of the lateral ocelli, and 

 five in the median one. This was determined by careful re- 

 constructions of transverse, frontal and sagittal sections of the 

 visual organ. In Eucalanus elongatus, Esterly found that the 

 lateral ocelli also contained nine retinal cells, but that the median 

 one possessed ten of them. 



