1891.] Zoology. 833 
which Parker asks is: ‘‘ What are the means by which ommatidial types 
are modified, and what is the significance of the changes through 
which these types pass?’’ The argument of the present paper is that 
the ommatidia composed of four cells are more primitive than those 
containing many cells, and to show how cell-division can have caused 
the modifications and what other factors may be concerned. 
Parker recognizes three types of compound eye so far as retinal 
structure isconcerned: I., represented by Decapods, Schizopods, Sto- 
matopods, Isopods, Leptostra, and Branchiopodidz of the Phyllopods, 
has the retina a simple thickening in the superficial ectoderm ; II., in 
Apodide, Estheride, and Cladocera, the retina does not retain a 
superficial position, but becomes covered by a fold of the ee 
the pocket remaining open in some and closed in others; III., 
Amphipods, and possibly Copepods, the retina is completely uke 
from the hypodermis, not by an infolding, as in the second type, but 
by delamination. Of this the author is not certain from actual obser- 
vation, but he gives a number of facts which warrant his conclusion. 
Types II. and III. pass through a stage comparable to type I., and 
hence may be regarded as derivatives from it. 
Detailed descriptions of the structure of the ommatidia in various 
forms are given and illustrated by numerous figures. Continuing his 
work with that of his predecessors Parker tabulates the ommatidia of 
the different Curstacea in a manner which conveys at a glance the 
difference between them so far as numbers of cells are concerned. In 
an ommatidium going from the surface to the optic nerve there are 
present (1) the cells of the corneal hypodermis which secrete the 
cornea; (2) the cone cells, varying in number from two to five, one 
function of which is to form the crystalline cone ; (3) the retinular cells, 
which may be either undifferentiated or divided into two groups, 
proximal and distal, the total number varying between three and 
nine; and (4) accessory pigment, cells which are either of ectodermal 
or mesodermal origin, all of the rest of the eye being clearly ecto- 
dermal. 
All compound eyes of the Crustacea are thus clearly built upon a 
common plan, and the variations between them are traced to (1) the 
differentiation of cells, (2) the suppression of cells, and (3) probably to 
multiplication of cells by division, although this last has not been 
demonstrated by observation. ; 
arker traces the origin of the ommatidium to such structures as are 
seen in the Chzetopod Nais, where there is an eye composed of a few 
large transparent cells, the distal portions of which are in part covered 
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