208 



THE EYE IN EVOLUTION 



Nauplius larva 



abundant animal constituent of the plankton. The group also contains some 

 parasites, as the common fish-louse, Caligus. 



{d) ciKRiPEDE.i — with an indistinctly segmented body and usually provided 

 with a calcareous shell. They have a complex life-history. They are born as 

 actively swimming nauplius larvae, develop into a pupal cypris-like stage, again 

 swimming freely with appendages, but in the adult condition lead an entirely 

 sessile or parasitic life. Typical examples are the barnacle, Lepas, which attaches 

 itself to the bottoms of ships or floating logs, the acorn-shell, Balanus, which 



Figs. 204 to 206. — The Eyes of Small Crustaceans 

 (Specimens from Natural History Museum, London). 



c 



X^^ 



i^ 



Fig. 204. 



Fig. 204. — The dorsal surface of a Branchiopod, Triops [Apus) cancri- 

 formis. In the anterior region are two compound eyes, C, and behind them a 

 median eye of the composite simple type, S. 



Fig. 205. — An Ostracod, Cypria ophthalmica. The single deeply jjigmented 

 eye, E, is seen shining through the semi-transparent shell. 



Fig. 206. — The water-flea, Daphnia. Prominently in the head region 

 (at the junction of the arrows) is the compound eye, ajDijearing as a mass 

 of pigment with little facets romid it. Behind and underneath lies the minute 

 composite median ej^e (see also Fig. 145). j 



encrusts the rocks between tidal marks in enormous numbers, and Sacculina, 

 ■.■!ie of the most degenerate of parasites which becomes an endoparasite in the 

 i.-' •' anen of crabs. 



of 



rhe characteristic ociilar feature of the whole group is the presence 

 uedian unpaired eye ; it is sometimes unique, as in Cyclops^ 



