No. 1.] EYES OF ARTHROPODS. 153 
base of the optic nerve develop into a large retinal ganglion. 
The main portion of the optic ganglion arises from the middle 
lobe. 
The similarity in the structure of the optic ganglion of these 
distantly related genera shows that the optic ganglion of the 
convex eye of Insects is homologous with that of Crustacea, and 
that in both groups its structure may be reduced to the same 
plan. Zhe variations in the structure of the optic ganglion of 
Arthropods may be referred to the modification, development, or 
suppression of one or more of three lobes, to which may be subse- 
quently added the retinal ganglion. What is the meaning of the 
three lobes so constantly present, at one stage or another, in the 
optic ganglion of Arthropods? It would be difficult to form a 
fruitful hypothesis of their significance from a study of forms 
that do not pass through an active larval existence, for we now 
know that the optic ganglion of the imago is derived from a larval 
ganglion, which in some cases is highly complicated, and has 
itself a long story to tell. In Astacus, Cymothoa, etc., the con- 
vex eye and its optic ganglion appear at a period corresponding 
with the end of the larval stage. Hence we must look for the 
preliminary stages of the optic ganglion in the larva and em- 
bryos of such forms as Acilius; and there we may expect to 
find a solution of the three-lobed structure of the ganglion of 
the convex eye. 
We have already seen how, in the earliest stages of Acilius, 
the optic plate is composed of three segments, each bearing a 
pair of eyes, and on the dorsal side of each segment there is a 
ganglionic invagination. 
The three masses of ganglion-cells thus produced unite to 
form the larval ganglion. Toward the close of larval life, each 
segment increases rapidly in size; the third segment grows 
around the proximal side of the ganglion to form the inner lobe, 
the second forms the fold around the anterior edge of the gan- 
glion, or the outer lobe, and the first forms the posterior inner 
mass of the ganglion, or the middle lobe of the future ganglion 
of the convex eye. 
At the beginning of pupal life, the ocelli are drawn towards 
the brain, and are replaced by the convex eye, which unites 
with the larval ganglion in a way that I do not yet understand. 
From these facts we may infer that the three-lobed optic gan- 
