302 



THE ANIMAL KINGDOM 



Compound 

 eye 



-Lenses 



Figure 16.11. Part of the head of Daphnia showing compound eye with protruding 

 lenses and muscles (M) of the right side attached to the side of the head (at A). Also shown 

 are the optic nerves {ON), optic ganglion (G) and brain. The nauplius eye (A'^) is de- 

 scribed in the text. 



is separated off, as in the crayfish, to form a pericardial sinus containing 

 the heart. Daphnia lacks arteries and veins. The heart pumps blood 

 forward, where it streams among the head organs, curves ventrally, and 

 flows posteriorly through the body organs. As in the crayfish the hemo- 

 coel also extends into the carapace. A coiled tubule on the antero- 

 ventral part of each side of the carapace is the shell gland, believed 

 to be an organ of excretion. 



Compound eyes arise embryologically as paired structures that later 

 fuse to form a single eye (Fig. 16.11). As it develops it sinks into the 

 head and is covered over by the exoskeleton, enclosing a cavity. Three 

 pairs of muscles from the sides of the head to the rim of the eye can 

 turn it in various directions. These muscles also keep the eye in con- 

 stant motion, jiggling it several times a second. Since the eye is com- 

 posed of only a few ommatidia, each of which gathers light from a 

 relatively wide area, this jiggling may improve vision (the human eye 

 has a microscopic jiggle, and our visual acuity is better than the struc- 

 ture of the eye alone would predict). Ominatidial lenses are large and 

 protruding. From the eye a bundle of optic nerves passes to a large 

 optic ganglion connected with a still larger brain. The circumesoph- 

 ageal connectives, subesophageal ganglion and the few ventral ganglia 

 are seldom visible. 



Attached to the antero-ventral margin of the brain is another un- 

 paired median eye, the nauplius eye (Fig. 16.11). This eye is found as 

 the only eye in nauplii, where it typically has a central pigment mass 

 with one anterior and two lateral groups of visual cells. It frequently 

 persists in adidt crustaceans. In Daphnia the anterior group is reduced 

 and divided into a single anterior cell and two ventral cells. Each 

 lateral group is reduced to a single postero-lateral cell. This eye is 

 suspended in the blood, its cells anchored by delicate fibers. The outer 

 ends of the cells turn back as nerves to the brain. This is the only in- 



