372 



CRUSTACEA EXTOMOSTRACA. 



appendages forms no exception to this rule. In Simoce phalus 

 and Sida among the Cladocera the ventral chain consists of 

 two longitudinal cords united by transverse commissures, and 

 giving off separate nerves to mandibles, first maxillae, and the 

 swimming appendages. In the adults of these genera the 

 central nervous system is not more than half the length of 

 the body, though in the young state the proportion is much 

 greater.* In Leptodora the ventral ganglia are fused into a 



common mass. The motor nerves 

 to the first antennae arise from, 

 or can be traced to (A pus) 

 the brain, those of the second 

 antennae arise from the oesopha- 

 geal commissures, or the sub- 

 oesophageal ganglion (Fig. 250). 



The Branchiopods possess a 

 pair of large eyes which are 

 frequently compound and some- 

 times fused together in the middle 

 line. In the Anostraca they are 

 stalked and movable (Sars). In 

 other forms, though superficial in 

 the larva, the eye is covered in by 

 a fold of skin which, growing 

 from behind, forms an open (Apus 

 and Estheridae) or closed (Clado- 

 cera) chamber over it (Grobben). 

 In addition a small median simple 

 eye (nauplius eye) may persist. 

 The first antennae bear olfactory 

 papillae in the Cladocera. 

 A group of sensory hairs in front of the median eye constitutes 

 the frontal organ ; and groups of ganglion cells in connexion 

 with the skin are found on the forehead (Branchipus) or at the 

 sides of the neck (Cladocera). 



Many members of the group have the power of attaching 

 themselves to surrounding objects by means of peculiar patches 

 of glandular cells situated on the dorsal surface (cervical gland), 

 near the fold separating the carapace from the head shield. 

 * Cf. Cunnington, Simocephalus, Jen. Zeits., Bd. 37. 



FIG. 259. Ventral aspect of the brain 

 of Daphnia similis (from Korschelt 

 and Heider, after Claus). c 1 and c 2 

 divisions of the supraoesophageal 

 ganglion ; c 3 suboesophagsal ganglion ; 

 go optic ganglion ; n nerve to the 

 sensory organ of the neck ; na' nerv tc> 

 1st antenna; n' and na" nerves to 2nd 

 antenna ; sc oesophageal commissure. 





