THE SESSILE-EYED CRUSTACEANS 291 



CHAPTER XX 



OEDER II. EDKIOPHTHALMA 



In this order tliere 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 Amphii^oda. 



Suh-order 1. — Cumacea. 



As with the Squillidae, 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 hefore 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 



