186 THE EYE IN EVOLUTION 



organ is formed as a modified tube-foot lying on a slight elevation (the 

 " optic cushion ") on the dorsal surface of the terminal ossicle (Fig. 166). 

 The organ is bright red due to the presence of ^-carotene and esterified 

 astaxanthin and consists of an aggregation of several cupulate ocelli of 

 the simplest type covered by cuticle and lined by sensory and pig- 

 mented cells (Plate I ; Fig. 167) (PfefiFer, 1901) ; a central lenticular 

 body may serve to concentrate light upon the receptive elements (van 

 Weel, 1935 ; Smith, 1937). The optical function of this organ in 

 Asterias has been convincingly demonstrated by Hartline and his co- 

 workers (1952) who recorded the electric impulses following stimulation 

 by light. The terminal tube-foot appears to be olfactory in function. 



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(1916). J. comp. iVewroZ., 59, 61 (1934). 



Hartline, Wagner and MacNichol. Cold Sarasin, P. and F. Ergebn. naturwiss. 



Spr. Harb. Symp. Quant. Biol., 17, 125 Forsch.Ceylon,V^\esh&den, 1, 1 (1887). 



(1952). Scheer. Naturwissenschaften, 43, 501 



Hertwig, O. and R. Jena. Z. Naturwiss., (1956). 



11, 355 (1877). Schewiakoff. Morphol. Jb., 15, 21 (1889). 



Das Nervensysteyn u. die Sinnesorgane Smith. Philos. Trans. B., 227, HI (1937). 



d. Medusen, Leipzig (1878). von Uexkiill. Z. Biol., 40, 447 (1900). 



Jourdan. Les sens chez les animaux Umwelt u. Innenwelt d. Tiere, Berlin 



inferieurs, Paris (1889). (1909). 



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39, 321 (1923). (Paris), 143, 1221 (1949). 



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Mem. Ser. 8, 10 (1900). (1935). 



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973 (1953). 



WORMS 



The large group of " worms " shows a variety of visual organs as 

 pleomorphic as the multitude of forms which constitute this loose 

 grouping of animals, showing every variation from a unicellular eye to 

 a relatively complex organ. In some cases the surface of the whole body 

 seems to be sensitive to light and it has not been possible to identify 

 specific sensory cells ; in most cases, however, specialized sensory 

 structures occur, for the elucidation of which we are largely indebted 

 to the classical work of Richard Hesse (1899-1908). Their presence, 

 their number, and the degree of their differentiation vary with the 

 animal's mode of life. This is the lowest group in the animal kingdom 

 to show l>ilateral symmetry and the sense organs share in this general 

 scheme distribution ; moreover, these organs are usually concen- 

 trated 1 rds the head-end of the animal where they are of greatest 

 biologic; lue. 



