120 MORPHOLOGY OF INVERTEBRATE TYPES 



of an anterior median region from which a nerve runs to the 

 ocellus or simple eye, a pair of lobi optici or protocerebrum with 

 optic nerves for the compound eye, and a pair of ganglia anten- 

 nularia or deuterocerebrum with nerves for the antennules. The 

 brain is dorsal to the oesophagus and sends out a right and left 

 connective to the subcesophageal mass. This mass consists of a 

 pair of ganglia with nerves for the antennae, and corresponds 

 therefore to the ganglia antennaria or tritocerebrum of higher 

 crustaceans in which it has become a part of the brain, while 

 the next pair of ganglia moved into its place to form a sub- 

 cesophageal mass. The subcesophageal mass and the following 

 chain of ganglia are ventral to the alimentary canal. This ven- 

 tral chain of paired ganglia is characterized by a great concentra- 

 tion of its elements. The last pair of ganglia belongs to the 

 somite carrying the fifth pair of thoracic feet. 



Daphnia pulex has a pair of olfactory organs in the antennules, 

 a simple eye, a compound eye and a great number of tactile 

 hairs. A tactile hair is a process of a modified hypodermal cell 

 connected through the intermediation of a terminal ganglionic 

 cell with the central nervous system. The olfactory organs of 

 the antennules may be regarded as modified tactile hairs. The 

 simple eye is of the cup type, reminding of similar structures in 

 lower invertebrates. The compound eye of Daphnia, while 

 originally of paired origin, becomes fused into a single organ 

 early in the embryonic life. It is more primitive than the com- 

 pound eyes of higher arthropods and cannot be very efficient. 

 To increase its usefulness it is provided with special muscles, the 

 alternate contraction of which keeps the eye in continuous mo- 

 tion to allow shadow-images to stimulate a greater number of 

 nerve endings. 



Reproductive system. The sexes are separate. In the 

 male the reproductive system consists of a pair of testes situated 

 one on each side of the midgut. The two vasa defer entia open 

 separately on the ventral surface of the trunk near its posterior 

 end. In the female the system consists of two long ovaries 



