CRUSTACEA. 



471 



region immediately be- 

 hind them is divided (fig. 

 215) into six segments 

 (the six thoracic seg- 

 ments) without appen- 

 dages, while somewhat 

 later the five anterior 

 abdominal segments be- 

 come indicated, but are 

 equally with the thoracic 

 segments without feet. 

 The mode of appearance 

 of these segments shews 

 that the thoracic and 

 abdominal segments de- 

 velop in regular succes- 

 sion from before back- 

 wards (Glaus). Of the 

 palp of the mandibles, 

 as is usual amongst Zoaea 

 forms, not a trace remains, 

 though in the youngest 

 Zoaea caught by Fritz 

 Muller a very small rudiment of the palp was present. The 

 first pair of antennae is unusually long, and the second pair 

 continues to function as a biramous swimming organ ; the 

 outer ramus is multiarticulate. The other appendages are fully 

 jointed, and the two maxillipeds biramous. On the dorsal 

 surface of the body the unpaired eye is still present, but on each 

 side of it traces of the stalked eyes have appeared. Frontal 

 sense organs like those of Phyllopods are also present. 



From the Protozoaea form the larva passes into that of a true 

 Zoaea with the usual appendages and spines, characterised how- 

 ever by certain remarkable peculiarities. Of these the most 

 important are (i) the large size of the two pairs of antennae and 

 the retention of its Nauplius function by the second of them ; 

 (2) the fact that the appendages of the six thoracic segments 

 appear as small biramous Schizopod legs, while the abdominal 

 appendages, with the exception of the sixth, are still without 



FIG. 215. PROTOZO.EA STAGE OF PEN^EUS. 

 (After Fritz Muller.) 



