SIGHT 41 
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 (Uvaster rubens) will 
* 
Hitec | 
Fig. 1052.—Rhopalia of Pericolga guadrigata, 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 ‘3 
radially symmetrical animals, and their eyes are _Fig. 1053.—An Eye-Cup 
. . . of a Star-Fish, greatly en- 
correspondingly disposed. But in “Worms”,  targed, in section. The 
Arthropods, Molluscs, and Vertebrates, where a ae 
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 Drrection- 
Eves, as they can do no more than detect the direction from 
which the light-rays which influence them are coming. Eyes of 
