CCELENTERATA 



183 



a primitive fiat eye, as in Turris or Lizzia (Fig. 96), or are invaginated 

 as an elementary cupulate eye, as in Sarsia (O. and R. Hertwig, 1878 ; 

 Jourdan, 1889). These organisms are light-sensitive and extirpation of 

 the tentacular bulbs with the ocelli completely abolishes the response 

 to light. 



Among the jellyfish (Scyphozoa) more elaborate organs are seen. 

 In the common jellyfish. Aurelia aurita, which is found in great shoals 

 around the British coast, eight sense-organs (tentaculocysts) arise 

 as modifications of tentacles ; each, lying in the protection of a marginal 



Figs. 163 and 1(54. — The Common Jellyj^sh, Acrelia aurita. 



Fig. 163. — Side view of the jellyfish, showing the 

 numerous marginal tentacles hanging from the 

 border of the convex umbrella, and the dependent 

 oral arms. The margin of the umbrella is broken 

 by 8 notches, the marginal lappets (L). 



Fig. 164. — A marginal 

 notch, showing a ten- 

 taculocyst comprised of 

 two olfactory pits, OP, 

 a calcareous concre- 

 tion, C, and an ocellus, 

 OC (modified from 

 Lankester). 



niche, has three types of sensory cells — red or black pigmented cells 

 responding to light, " olfactory " cells with a chemical appreciation, 

 and club -like cells containing calcareous concretions with a balancing 

 function (Figs. 163-4). 



Exceptionally, as in the Cubomedusan, Charybdea, a large ocellus has been 

 reported with a cellular lens, a vitreous structure and a complex retina — an 

 organ structurally capable of some degree of visual imagery (Fig. 102) (Schewia- 

 koff, 1889 ; Berger, 1898). The biological value of this elaboration in a brainless 

 organism is somewhat speculative. 



ECHINODERMATA 



Among ECHiNODERMS (" spiny skinned "), a phylum characterized 

 by its radial symmetry, visual organs are rudimentary. This would 



