EYES OF CRUSTACEANS IO5 



lateral compound eyes are present at one period of its development and 

 both have disappeared in the fully developed adult animal. In Apus and 

 Lepidurus both types of eye are retained in the adult animal, as in Limulus, 

 and it appears probable that many types of Crustacea, which in the adult 

 state are parasitic or becoming attached by a pedicle to rocks, lead a fixed 

 existence, have descended from ancestors who possessed both median and 

 lateral eyes and made use of them in their free-swimming adult life. 



In the Cladocera, as previously mentioned, the paired lateral eyes 

 have fused into a single median organ, as in Daphnia, Polyphemus, and 

 Leptodora (Fig. 21, p. 27), while in Cyclops, the water-flea (Fig. 71), it 

 is the simple, median pair of eyes which have fused into a bilobed eye-spot, 

 no compound eyes being developed in this form. 



In the parasitic Eucopepods, or fish lice, both median (simple) and 

 lateral (compound) eyes are present. The median eyes in the adult 

 animal are represented by a composite organ which appears as a three- 

 lobed structure in the median line, considerably behind the plane of the 

 two compound eyes. In the centre is a Y-shaped pigmented area which 

 separates three clear lens-like bodies. The anterior of these is probably 

 formed by the complete fusion of two eyes, while the fusion of the hinder 

 pair is incomplete. The arrangement is somewhat similar to the fusion 

 of ocelli which takes place in Sagitta hexaptera (Fig. 55, p. 86), but in 

 Sagitta the ocelli which have fused, presumably correspond to the lateral 

 paired groups of ocelli in the larval stages of higher types. In the 

 Phyllosoma larva {Palinurus) (Fig. 72) the median eye is bilobed, and it 

 is probable that one of two median pairs of eyes has degenerated or 

 completely disappeared. In this crustacean there is a very high degree of 

 development of the stalked compound eyes and optic ganglia. 



The structure of such a compound eye is well seen in Caprella acuti- 

 frons, belonging to the Order Amphipoda. If a whole specimen is viewed 

 from above under a low power of the microscope by transmitted light from 

 below, it presents the appearance shown in Fig. 73, and if the equator 

 of the eye is focused each retinula is seen to be covered by a transparent 

 corneal or cuticular facet. The central ommatidia show a star-like, 

 five-rayed, central axis, which transmits the light upwards from below. 

 Each of these stars corresponds to a retinula with its central rhabdite and 

 crystal cone (see Fig. 44, showing longitudinal and transverse sections of 

 the eye of Gammarus ornatus, the fresh-water shrimp, a nearly allied 

 species). Surrounding the clear area are five pigment cells filled with 

 black pigment, while the intervening spaces are occupied by interstitial 

 tissue, and in Gammarus also by the white pigment cells already referred 

 to (p. 65). 



The compound eyes and optic ganglia of the crayfish, Astacus 



