ITS EYES. 175 



the ray-tip, and also the water in a horizontal direction ; 

 and as there is one at each of the five angles, the entire 

 circuraferent space is viewed at each moment. Add to 

 this, that the animal is in the habit of very frequently 

 turning up the tip of one or other of its rays, when the 

 range of vision would take in the zenith ; and we shall 

 perceive that no position in any other part of the body 

 could be so suitable for the location of eyes as these 

 selected. It is not to be supposed that distinct defi- 

 nition of objects is attained by these rudimentary 

 organs ; but the animal is probably conscious of the 

 difference between light and darkness, and may also 

 discern the sudden approach of any object, either by 

 its interception of light, or by its colours, though its 

 features and form may be indistinct. Such a degree of 

 visual perception, though very imperfect in our esteem, 

 may be of great use to this sluggish creature, and amply 

 sufficient for its need. 



The whole upper surface of this Starlet is covered 

 with six-sided convex plates, each of which is crowned 

 by a group of short blunt spines ; the number in each 

 group varying from one to nine ; four to six, however, 

 most commonly ; arranged in a diverging tuft. Over the 

 madreporiform tubercle, the tufts are stouter, and bend 

 towards each other, as if to protect this delicate organ, 

 which, whatever its function, is grooved with sinuous 



