THE SESSILE-EYED CRUSTACEANS 29] 
CHAPTER XX 
ORDER IIl.—EDRIOPHTHALMA 
In this order there is generally a pair of compound eyes, 
which are sessile ; they may be prominent, but are never 
movably stalked ; they may be absent, or placed so closely 
together as to form apparently, but not really, a single 
eye. On the other hand, the visual elements may be 
variously distributed, so as to form compound eyes from 
one to four in number, or simple eyes that are not limited 
to four. The last seven segments of the trunk are gene- 
rally not, and the last four are never, included in the cara- 
pace. 
Three sub-orders are comprised in this order—the 
Cumacea, Isopoda, and Amphipoda. 
Sub-order 1.—Cumacea. 
As with the Squillidee, so with the Cumacea, nothing 
is to be found about them in the writers of antiquity, but 
about the latter sub-order even the sixteenth century 
knew no more than its predecessors. Not a single species 
was recorded before the year 1779, and for sixty years 
after that date science contented itself with half a dozen 
species, some of which were so obscurely or imperfectly 
described that they are still doubtful. Since 1841, by 
the exertions of a few naturalists, chiefly Scandinavian 
and Anglo-Saxon, the characters of the group have been 
investigated with great thoroughness. The distribution 
extends to all oceans, and from tide-marks down to very 
great depths. By rapidly repeated discoveries the known 
forms have now become numerous, and are classified in 
