734 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Cells of 
Corneal 
Hypodermis. 
Cone- 
Cells. 
Retinular Cells. 
Accessory 
Cells. 
Undiffer- 
entiated. 
Differentiated. 
Proximal. 
Distal. 
1. Amphipoda .. 
pr. 
2 
5 
pr. (ect. ?) 
2. Branchiopodidse 
\ 2 
4 
5 
A 
and Apusidse 
J 2 
u 
3. Estheriidas .. 
pr. 
5(4) 
5 
0 
4. Cladocera 
V 
5 
5 
pr. (ect. ?) 
5. Copepoda — 
Fontella 
pr. 
2 
5 
pr. (ect. ?) 
Sapphirhma . . 
? 
? 
3 
? 
Argulus 
pr. 
4 
5 
? 
6 . Isopoda — 
Idotea . . 
2 
2 
6 
pr. (ect. ?) 
Porcellio 
2 
2 
7 
4 
2 
Serolis . . 
2 (+?) 
2 
,, 
7. Nehalise.. 
2 
4 
7 
8 . Sehizopoda . . 
2 
2 
7 + 1 
2 
+ pr. (mes. ?) 
9. Stomatopoda .. 
2 
4 
7 + 1 
2 
„ 
10 . Decapoda 
2 
4 
7+ 1 
2 
” 
The author thinks that the type from which the ommatidia of all 
living Crustacea are probably derived would exhibit the following 
structures, a corneal hypodermis in which the cells are not regularly 
arranged, and the corneal cuticula was not facetted ; a cone composed of 
two cells ; a retinula composed of five retinular cells, and a rhabdome con- 
sisting of five rhabdomeres. The retina of the primitive eye, a simple 
thickening in the superficial ectoderm, would be composed of ommatidia 
of this type, arranged upon the hexagonal plan. No known Crustacean 
has an eye of exactly this structure, but that of Gammarus seems to most 
nearly represent it. 
If these conclusions are correct, the principal types of ommatidia 
must have been produced mainly by increasing the number of cells in 
the primitive type ; the most influential means of modifying the structure 
of the ommatidia must have been cell-division. 
Dermal Sense-organs of Crustacea.* — Dr. O. vom Rath gives a 
preliminary account of his comparative observations on the sensory 
organs of various Crustacea. He has discovered sensory hairs on 
almost all parts of the body. The first pair of antennae are the bearers 
of the most important ; some act as protecting setae to the olfactory 
organs ; these are so attached to the cuticle as to be incapable of any 
great power of movement ; they appear to be closed by membranes so 
thin as to allow of delicate sensation and the passage of fluids. Un- 
feathered, half feathered, completely feathered, and toothed sensory hairs 
may all be found on the first antennae. The organs on the second pair 
are far less important than those on the first, though tactile hairs are 
often abundant, and exhibit great variation in size and shape. The 
gnathites always carry a number of sensory hairs, which the author 
regards as tactile bristles. 
* Zool. Anzeig., xiv. (1891) pp. 195-200, 205-14. See Arm. and Mag. Nat. 
Hist., viii. (1891) pp. 299-313. 
