SIGHT 



a number of compound sense - organs (rhopalia) derived from 

 tentacles, and having to do with balance and adjustment of move- 

 ments, sight, and possibly smell. Their visual part consists of a 

 group of pigmented ectoderm cells, upon which a lens may rest 

 {fig. 1052). 



Examination of a Common Star-Fish (Uraster rubens) will 



O D 



Fig. 1052. Rhopalia of Pericolpa quadrigata, seen from various points of view, enlarged. The otocyst, 

 containing numerous otoliths, is seen in the lower part of A, B, and c; the rounded pigmented eye, with clear, 

 central, refracting portion, is indicated in A, c, and D. 



reveal the presence of a bright-red spot at the tip of each arm, 

 borne upon an unpaired tube-foot. This is undoubtedly an eye, 

 and microscopic examination shows that it is made up of a multi- 

 tude of little cups, each of which is lined with elongated cells, some 

 of which are sensory, while others contain pigment (fig. 1053). 

 Sea < Urchins possess a circlet of somewhat 

 similar eyes placed near the upper pole of the 

 body. In some of these animals each of the 

 minute cups may be provided with refracting 

 structures, which presumably concentrate the 

 light. 



Jelly- Fish, Star- Fish, and Sea- Urchins are 

 radially symmetrical animals, and their eyes are 

 correspondingly disposed. But in " Worms ", 

 Arthropods, Molluscs, and Vertebrates, where 

 the body is bilaterally symmetrical, and there is 

 a more or less well-developed head, the eyes are usually situated 

 upon this, as being the most useful position. But eyes may be 

 present elsewhere, especially in some of the Planarian Worms, 

 and certain Bivalve Molluscs. 



The visual organs so far described may be called DIRECTION- 

 EYES, as they can do no more than detect the direction from 

 which the light-rays which influence them are coming. Eyes of 



Fig. 1053. An Eye-Cup 

 of a Star-Fish, greatly en- 

 larged, in section. The 



