No. 2.] EVES OF ARTHROPODS. 267 
spoken of. The mere fact of invagination! must be regarded 
as indicating an ancestral condition, but what this condition was 
is uncertain. The pit or groove must have had sensory func- 
tions and either wall” (retinogen and gangliogen) “must for a 
time have been like its fellow, as is shown by its having similar 
nuclei, and by the similar development of rows of nuclei.” 
In the species Mesothyra Oceant, Hall, a member of one of the 
faunas of the Portage group, and one of the largest known repre- 
sentatives of the Phyllocarida, the eye consists of a simple deep 
pit at the summit of the optic node. There is no evidence that 
this pit contained a series of lenses, but it is highly probable that 
it is an otherwise embryonic character retained at maturity, and 
may serve as the ancestral condition of the Decapod eye sug- 
gested by Dr. Kingsley. That there is Decapod blood in the 
Phyllocarida has been disputed by Packard, the author of the 
group term, but Decapod affinities are strongly indicated by the 
recently described Devonian genus of Phyllocarida, Rhinocaris, 
(Paleontology of New York, Vol. VIL, 1888). 
1 This refers to the primary optic invagination in the embryo, for the formation of 
the entire visual surface, first pointed out by Locy (Bud/. Mus. Comp. Zodl. 1886), 
and verified by Kingsley (of. cz¢.). The term is not used in a sense similar to that 
in which it has been employed in this paper in referring to the formation of the 
corneal lenses. 
