299 
1916-17.] Experiments and Observations on Crustacea. 
concerned in defsecation is more feeble in this animal than in Gammarus 
or in Ligia. In Idotea, and presumably in Glyptonotus, fsecal pellets are 
shot backwards to a distance by the current caused by the pleopods. 
The Eyes and the Chromatophore-System. 
The eyes of Glyptonotus are of interest. On first examination they 
appear to be entirely dorsal in position. In both species, however, each eye 
is divided into two quite separate parts, one lying on the dorsal the other 
on the ventral surface. 
Eights (1833) described their situation thus : “ Eyes . . . placed near 
the lateral and anterior margin of the head, so deeply impressed in the 
margin of the shell as to be easily distinguished from beneath.” Pfefier 
(1887) detected the true ocular nature of the ventral pigmented spot thus 
indicated by Eights ; his account is as follows : “ The eyes are situated, 
as the systematic diagnosis states, on the surface of the head, while in the 
genus Idotea they are situated on the border. The morphological relation 
between these two conditions may be conceived as follows : In all isopods 
a narrow border tends to run round the whole periphery of the animal ; so 
also in Idotea, in which the border is continued midway across the eye 
without interfering with the power of vision in this region ; for the border 
is here transparent and participates in formation of the cornea. In this 
way an Idotea can see in the upward direction, horizontally and down- 
wards. In Glyptonotus the transparency of the border has been lost ; the 
border, in this case stout and strongly pigmented, courses right across the 
eye as in Idotea. Thus the animal is deprived of vision in the horizontal 
direction ; above and below, the skin over the eye has remained trans- 
parent. Consequently Glyptonotus has one eye on the dorsal aspect of 
the head and another on the ventral aspect ; the latter has a true cornea, 
even if less regular and distinct than that of the upper one.” Though 
Pfeifer’s description is somewhat roundabout, I have quoted his observa- 
tions for the sake of his comparison with the eye of Idotea. 
In Ghiridotea the eye is entirely dorsal, a notch in the cephalic carapace 
denoting where the eye once extended over the lateral margin. The extinct 
Proidotea had likewise a notched cephalic carapace, the dorsal eye in this 
case being apparently situated somewhat nearer the lateral border than in 
Ghiridotea — see Racovitza and Sevastos (1910). Whether Proidotea had 
in addition a ventral eye is not known. 
Physiological experiment sheds a suggestive light upon these structural 
peculiarities, for Y. Bauer (1905) in experiments on colour change in 
Idotea came to the conclusion that the apparently single eye of this animal 
