ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
733 
again, the eyes are always on the ventral side of the lino of demarcation, 
and on this line there are always spines. 
The nervous system arises from a paired longitudinal thickening of 
the ectoderm ; the anterior are much broader than the posterior ends, 
and there are two pairs of ectodermic invaginations ; these parts form 
the brain. The ganglia are separated gradually from the general 
ectoderm, and this separation is effected before that of the brain. The 
eyes are developed from pre-oral ectodermic invaginations, externally to 
the brain ; they are produced at the margin of the ventral plate and 
retain this position ; the lateral eyes migrate backwards, and it is this 
movement which has led many authors to suppose that they belong to a 
thoracic segment. 
The median eyes arise from a pair of small ectodermic invaginations, 
which are afterwards united into a tube ; this tube is subsequently 
reduced to a solid rod, the distal end of which is enlarged, and lies at 
the margin of the ventral and dorsal surfaces. 
e. Crustacea. 
Compound Eyes of Crustacea.* — Mr. G. H. Parker has an elaborate 
memoir on this subject. The principal question which he put before 
himself was, “ What are the means by which ommatidial types are 
modified, and what is the significance of the changes through which 
these types pass.” He has himself suggested already that those 
ommatidia which are composed of a small number of cells more closely 
resemble the ancestral type than those composed of many cells. 
Three retinal types are distinguished in the compound eyes of Crus- 
tacea. In one the retina is a simple thickening in the hypodermis ; this 
type is characteristic of Isopods, Branchiopodidae, Nebaliidae, Stomato- 
pods, Schizopods, and Decapods. In the second type the ectodermal 
thickening becomes inclosed within an optic pocket ; this may remain 
permanently open, as in the Apusidae and Estheriidae, or may become 
closed as in the Cladocera. In the third type the retina originates from 
thickened hypodermis, which subsequently separates into the corneal 
hypodermis and the retina proper. This is seen in Amphipods and 
Copepods. 
The author thinks that the course of development taken by each of 
the three types very clearly indicates their mutual relations. The first 
of the types is evidently primitive, and as the other two pass through it 
they may be supposed to have been derived from it. In each case the 
retina is fixed in the simpler and movable in the more differentiated types. 
The author very conveniently sums up his knowledge of the cellular 
composition of the ommatidia in the table given on the next page, 
wherein the abbreviation pr. marks the presence of any kind of cell 
when the number of that kind is not constant for different ommatidia in 
the same individual. In the Estheriidae cones with four cells are some- 
times found, though five is the usual number. It is possible that in 
Serolis there may be more than two cells in the corneal hypodermis of 
each ommatidium. In Schizopods, Stomatopods, and Decapods, the 
eighth proximal retinular cell is rudimentary. 
Bu*ll. Mus. Comp. Zool., xxi. (1891) pp. 45-140 (10 pis.). 
