NERVOUS SYSTEM. EYES. 347 



allusion lias just been made) abdominal legs are not found in 

 the Entomostraca, but in the Malacostraca they are nearly 

 always present as biramous swimming limbs (pleopods), the 

 posterior pair (uropods) often forming with the telson the 

 powerful caudal fin. In Isopods and Stomatopods the respira- 

 tory function is carried on by the abdominal appendages. 



The central nervous system. In the Phyllopoda (Fig. 241 D) 

 the ganglia of the ventral chain are wide apart and each is con- 

 nected with its fellow by double transverse commissures. There 

 is a pair to each pair of postoral appendages. Many of the 

 lower Malacostraca present little advance on this arrangement, 

 but various degrees of concentration are presented by the several 

 groups. 



The composition of the supraoesophageal ganglionic mass 

 differs, as pointed out above (p. 323), in the Malacostraca and 

 in some members at least of the Entomostraca. In the former 

 there is evidence that three neuromeres join with the optic ganglia 

 to form the brain. The middle and the posterior neuromeres 

 give off nerves to the first and second antennae respectively. 



In the Branchiopod genera Daphnia and Limnadia the neuro- 

 mere of the second antennary segment retains its post-oesopha- 

 geal position (Figs. 241 D and 250) and forms the first ganglion of 

 the ventral chain. Hence we have in recent Crustacea two stages 

 of the process by which the compound brain of Arthropods is 

 formed ; one (represented by Daphnia and Limnadia) in which 

 the second antennary neuromere is post-oesophageal, the other, 

 the malacostracan stage, in which it has become pre-oral, and 

 merged in the mass of the brain, although in many Malacostraca 

 the paired ganglia forming it are apparently connected by a 

 transverse commissure (Fig. 241 y] passing behind the oesophagus. 



Frontal sense organs. Attention may here be drawn to small 

 paired sense organs, found in many Entomostraca on the front 

 of the head. In the larva of Apus they consist of papillae 

 with dilated bases. 



The unpaired or median eye often called the nauplius eye 

 is a characteristic feature of the Crustacea, occurring alone or in 

 association with the compound eyes in all the groups of the 

 Entomostraca, and in the larval stages of the Schizopoda, 

 Decapoda and Stomatopoda among the Malacostraca. In some 

 Copepods it consists of 3 groups of cells, a median ventral and 



