212 THE EYE IN EVOLUTION 



borders of the visual cells unite with their neighbours to form rhabdomes 

 (Fig. 109) ; the median eyes are also of the simple type with the 

 sensory cells arranged in groups each centred on a rhabdome. These 

 cells, however, are peculiar in that they are doubled upon themselves 

 to form a semi-inverted retina ^ (compare Fig. 127). ^ 



The KING-CRABS (xiphosuea), a very ancient type dating to the 

 Silurian, w^hich live in shallow water on the sandy shores of North 

 America {Limulus) or Asia, have two large lateral compound eyes and 

 two median ocelli (Fig. 211). The compound eye is of a unique and 

 elementary type ^ (Fig. 143) ; it is not faceted but is covered by a 

 chitinous thickening of the cuticle which sends projections inwards as 



Ki( 



'I'll i: .1 I Mi'iN(. Si'i 1 'i:i!, /'/ / A //■ 



SIM'ATL'f!. 



From the Dutch East Indies. The 2 large and 6 small simple eyes are 

 seen surrounding the anterior and lateral aspects of the carapace (specimen 

 from Natural History Museum, London). 



conical papillse over each ommatidium to form a corneal lens. The 

 small median eyes are of the simple type wherein the sensory cells are 

 associated with rhabdomes (Fig. 142). 



In addition, a third pair of ventral eyes is present in the larva on 

 either side of the frontal organ of the hypostoma. an olfactory organ ; 

 in the adult these eyes become degenerate but it is possible that they 

 may participate in the olfactory function (Patten, 1893 ; Hanstrom, 

 1926). 



2 F ' details, see J. Miiller (1826), Lankester and Bourne (1883), Parker (1887), 



Petrun! -li (1907), Police (1908), Scheuring (1913-14). Biitschli (1921), Versluys 



and Deu: 1923). 



"p. 



