ARTHROPODA — CRUSTACEA 299 



phakos, lentil (lens), -\-ops, eye, refers to the many lens-like 

 facets of the compound eyes. 



This genus is abundant throughout the world. P. rana was 

 abundant in the sea covering eastern North America during the 

 middle of the Devonian period. 



1. Sketch {a) specimen; {b) entire eye. Note glabella, facial 

 sutures, eyes, eye facets, pygidium, pustulose surface. 



2. What part of the entire animal does this fossil represent ? 



3. What is the significance of the name? 



4. What differences between the eyes of Phacops and those 

 of Calymene ? 



5. How does the glabella of Phacops differ from that of 

 Paradoxides ? from that of Calymene ? 



6. How does the relation between fixed cheek, facial suture 

 and genal angle differ in Phacops, Calymene and Paradoxides ? 



7. From analogy with a living crayfish describe how Phacops 

 molted. , 



Sub-class 2, Phyllopoda 



Small, with elongate body segmented and covered anteriorly 

 with a carapace. The great majority inhabit fresh water ; 

 Artemia lives in salt lakes and brine pools. Many produce two 

 kinds of eggs, the parthenogenetic thin-shelled and the sexually 

 produced thick-shelled ones, which latter alone can survive cold 

 and desiccation. Known in the fossil state from the Cambrian 

 to the present. 



Derivation of name. — Greek phyllon, X^Siij + pons {pod), 

 foot. So named from the broad, flat, leaf-like feet. 



Apus (Fig. 132). Triassic to present. 



Apus is one of the most primitive of all hving Crustacea; 

 it is present, though not common, in the fresh waters of nearly 

 every portion of the world and is found in the fossil state from 

 the Triassic to the present. 



In general appearance it differs from the crayfish in its 

 broadly projecting oval carapace, in the pair of long processes 

 at the end of the anal segment, in its small size (it is an inch or 



