ESTERLY : EUCALANUS. 33 



fortsetzt." Grenacher ('79) makes a similar statement. Both Claus 

 ('91) and Hesse (:01, :02), however, state that the median eye of 

 Crustacea lies outside the ectoderm. 



]\Iv preparations show that this statement quoted from Claus ('91) 

 is only partly correct. The optic nerve is surrounded by a very dis- 

 tinct membrane, which becomes. blue in jNIallory's stain, and is there- 

 fore to be regarded as composed of connective tissue, and is probably 

 of mesodermal origin. The membrane enveloping the optic nerve is 

 continuous with the one around the brain (Plate 1, Fig. 4), but has 

 nothing at all to do with the neurilemma of the nerve fibres (Plate 5, 

 Fig. 47). But the discussion of the relation between this sheath of 

 the optic nerve and the eye may be deferred for the present. 



In its relation to the h^^Dodermis, or ectoderm, the ventral part of 

 the median eye corresponds in every way to the first of the retinal 

 t\^es mentioned by Parker ('91), though Hesse (:02) states that the 

 whole median eye is detached from the ectoderm. Both cross and 

 sagittal sections of the eye (Plate 1, Figs. 3, 6; Plate 5, Figs. 44, 

 48) show with perfect clearness that the ventral portion of the trip- 

 artite eye is merely a thickened region in the ectoderm (h'drm.), and 

 has maintained its superficial position permanently. The h\'podermis 

 and retinal cells are adjacent ; in other words, the retina and ectoderm 

 are continuous. The basement membrane of the ectoderm may be 

 traced in sagittal (Plate 5, Figs. 44, 48) and in cross sections (Figs. 

 49, 50) continuously from a region entirely outside the retina, over 

 (dorsal to) the basal plate of the eye, and on to the ectoderm as such 

 again. This basement membrane is very delicate, but visible with 

 perfect clearness in any preparation, though jNEallory's stain shows it 

 most distinctly. In this stain the membrane does not become blue, 

 and so should be regarded as of a different nature from that envelop- 

 ing the optic nerve and brain. 



Such nerves as leave the ventral eye through the basal plate, must 

 penetrate the basement membrane of the hypodermis as well as the 

 basal plate, since the plate is a product of the cells composing the 

 retina, and these are plainly specialized ectodermal cells. The nerve 

 fibres which in passing from the visual cells do not penetrate the basal 

 plate pass through the basal membrane only. 



The relations of the dorsal components of the eye and their envelop- 

 ing membranes are not so easily made out as in the case of the ventral 

 eye. Each of the paired eyes is surrounded by a delicate sheath 

 (Plate 5, Figs. 46, 50), but in the adult condition it is evidently not 

 related to the ectoderm as in the ventral eve. Such illustrations as 



