No. I.] EYES OF ARTHROPODS. 167 
shown, however, that the appendage is not, strictly speaking, 
an outgrowth of the posterior dorsal ocellus, as I formerly sup- 
posed, and I do not now see any reason to suppose that the 
ocellus proper develops into any part of the compound eye. The 
appendage is, I now believe, one or two of the primitive sense 
organs of which the ocellus is composed, that have not com- 
pletely united with the others, and have undergone a special 
modification in the direction we have already explained. 
Toward the close of larval life, the convex eye appears as a 
thickening of the ectoderm immediately around the appendage. 
At this time it is difficult to distinguish any line of demarkation 
between the appendage and the thickening. In the latest stages 
I possess, it forms an enormous, thickened band that almost en- 
circles the six ocelli. The band is narrow in the middle, but 
expanded and rounded at either end. Its ventral edge is deeply 
invaginated, especially near the appendage where the invagina- 
tion first appears, and is connected with the adjacent ectoderm 
by a thin vertical layer of cells (Fig. 1, wood-cut). At first it 
seemed probable that the appendage developed into the ventral 
half of the convex eye, and the thickened band into the dorsal 
half. If the band itself should be divided into two parts, this 
interpretation would be, so far as I can see, untenable, and I 
must admit that there is an indication of such a division, though 
so indistinct and unaccompanied by any difference in the de- 
velopment of the ommatidia on either side of it, that I am still 
in doubt as to its meaning. 
It is difficult to believe that the appendage of the posterior 
ocellus has nothing to do with the convex eye, since the lat- 
ter is, in the early stages, so intimately connected with the 
former. One thing is certain, that a great part of the com- 
pound eye arises suddenly at the close of larval life as a thick- 
ening in a previously indifferent layer of hypodermis; hence 
that part at least cannot be considered a modification of any 
functional larval organ. 
The development of the frontal ocelli points to the conclusion 
that they are widely different from the larval ones, and perhaps 
closely related to the compound eyes. 
One of the things that impressed itself most deeply upon me, 
after studying the embryology of Acilius, was the threefold 
structure of the head as shown in the three segments of the 
