MUSEUM OF COMPARATIVE ZOOLOGY. 197 
scorpions is unquestionable, yet whether the lateral eyes, like the me- 
dian, have been formed by an involution with inversion, or whether 
their formation is accompanied simply by a thickening and more or. 
less extensive depression in the hypodermis, is still an open question. 
Graber’s figure (’79, Pl. V. fig. 4), with its preretinal membrane and 
lentigen, would indicate that the eye arose by involution. Lankester and 
Bourne’s figures (’83, Pl. X. figs. 2, 3, and 4), in which these structures 
are absent, would favor the explanation that the eye is only a hypodermal 
thickening. 
The position of the lateral eyes in scorpions has already been described. 
In the adult Centrurus each group consists of four eyes, three of which 
are large and are designated by systematists as “ principal” eyes, and 
the fourth is small and known as an “accessory” eye. The larger eyes 
are arranged in a horizontal line at the antero-lateral angle of the shield ; 
the small eye is above a point midway between the posterior and middle 
larger eyes. 
A vertical section through the axis of one of the larger eyes (PI. ILI. 
fiz. 18) shows at the surface a strongly convex lens (/ns.) beneath which 
a relatively small retina (v.) appears. The outline of the latter is marked 
by the basement membrane (mb.), and on its dorsal and ventral edges it 
is seen to be continuous with the hypodermis (hd.). In an eye from 
which the pigment has not yet been removed, the whole retina is 
intensely black. The pigment extends up to the margin of the lens, as 
ficured by Lankester and Bourne (’83, Pl. X. fig. 1), and spreads out 
above and below into the adjacent hypodermis. It is far more abundant 
in the dorsal hypodermis than in the ventral. 
The Zens in the adult eye consists of essentially the same parts as in 
the median eye, and contains no pore-canals. Its substance except the 
front hyaline layer is stained throughout by alcoholic borax-carmine. In 
young individuals (Pl. III. fig. 21) the lenses of the lateral eyes, even 
better than those of the median eyes, show a formation of stainable 
euticula (di) under the hyaline layer (//) before a similar secretion has 
taken place from the general hypodermis. 
In the adult eye not the least appearance of a lentigen or preretinal 
membrane is to be found, even after careful depigmentation. The fact 
that the pigmentiferous tissue extends up to the lens is of itself sug- 
gestive of the absence of a lentigen, for in ocelli generally this layer is 
remarkable for its transparency. When to this is added the fact, that no 
nuclei exist in the front part of the eye, and that in no place does the 
basement membrane extend as a preretinal membrane across the front of 
