EEPOET ON THE ANNELIDA. 
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to consist of a central cavity surrounded by two coats. The central space is partially 
occupied by a faintly granular, transparent substance of firm consistence, probably of the 
nature of vitreous. The outer of the two coats is the above-mentioned prolongation of 
the cuticle ; the inner covering represents the retina and its central connections. 
1. The Outer Coat. — At the periphery of the globe, behind or at the outer side, this 
coat may be seen to be formed of two distinct layers. From without inwards are 
found ; — 
(а) A thin transparent chitinous layer. 
(б) A fine connective tissue stroma with distinctly nucleated epithelial-like cells 
(see PI. XXXIIIa. fig. 2). 
It is doubtful whether or not this layer is again followed by a thin, clear, internal 
covering ; in one section this seems to be the case.^ 
Tracing this outer coat forwards over the eyeball, we find that it becomes very thin, 
homogeneous, and transparent opposite the middle of the globe. It appears to consist 
here merely of the chitinous layer somewhat thickened, and lined internally by a delicate 
endothelium^ (PI. XXXIIIa. figs. 1, h, and 3). Still more anteriorly this coat again loses 
its transparent, homogeneous appearance, and in the middle line it meets the correspond- 
ing coat of the opposite eye at an acute angle and becomes blended with it, forming here 
a thick layer with numerous spaces seen on section (PI. XXXIIIa. figs. 1 and 4, a). 
Some of these spaces, of well-defined elongated oval form, are evidently sections of 
blood-vessels. This outer coat must be considered as strictly analogous with the 
corneo-sclerotic of higher animals, the thin, transparent, central part representing the 
cornea. 
2. The Inner Coat. — The retina and its central connections. 
What first attracts the attention in this situation is a layer of reddish-brown pigment. 
Tracing it from before backwards, we find it first lining the inner surface of the anterior 
part of the sclerotic, beginning just outside the corneal margin. It is continued back- 
wards in this relation until it meets with a structure projecting from the fundus well into 
the interior of the globe, over the anterior or inner surface of which it is reflected. At 
intervals this pigment is disposed in little heaps, but there is no definite arrangement of 
the aggregations as we find in the Alciopidse. It consists of minute round granules, each 
about 0'5 mm. in diameter. 
The mound-like structure just referred to as projecting from the fundus, contains 
numerous ganglion cells, fine molecular material, and exquisitely delicate fibrils. From 
its position we should accept it as the nervous part of the retina, but it really represents 
1 Compare R. Greeff, Untersuchungen tiber die Alciopiden, Nova Acta Acad. Gees. Leap., Bd. xxxix., No. 2, p. 96. 
“ This endothelium is probably of the same nature as that described by Greeff in this position in the Alciopidae, and 
traced by him backwards to the periphery of the cerebral ganglion, with the covering of which it is directly continuous, 
op. cit, p. 97. 
