250 ORGANS OF THE SENSES. 



most other radiated animals render less necessary in them 

 the development of organs of this nature, and they have not 

 been detected in any of the poripherous or the polypipherous 

 animals, unless the bright coloured opaque points seen on 

 the disk of the locomotive actiniae, and of some other polypi 

 are organs destined to absorb and to communicate impres- 

 sions of light. In the acalepha, organs of vision have yet 

 been observed only in the medusa aurita, where they are mi- 

 nute round points on the dorsal side of eight brown globules, 

 placed on little peduncles, in the eight depressions around 

 the free edge of the mantle, and they have the same red co- 

 lour as those of many other transparent animals as polygas- 

 trica, rotifera, and entomostracous Crustacea. These eight 

 small, red, pedunculated eyes directed upwards, are provided 

 each with a crystalline lens, an optic ganglion, and two de- 

 cussating optic nerves derived from the exterior circle of 

 nerves which accompany the marginal canal of the mantle, 

 and they are placed near the bases of the marginal tentacula, 

 like the numerous marginal pedunculated shining eyes of a 

 spondylis or a pecten among the conchifera. The organs of 

 vision in the echinoderma, as well as those of the acalepha, 

 have been long figured by authors, though their nature has 

 been till lately overlooked. Beneath the distal extremity of 

 each radiating division of the body of the asterias violacea 

 and asterias militaris, a small, circumscribed, round, red 

 coloured, retractile point is observed, as represented by Vahl, 

 which rests upon a small optic ganglion at the end of each 

 of the five radiating nerves. These are analogous in their 

 position and characters to the common visual organs of ani- 

 mals thus low in the scale, and probably will be found on 

 holothurise and other active echinoderma, where the nervous 

 system is most developed. They yet present no transparent 

 parts besides the cornea which has sometimes a glistening or 

 shining appearance even in polygastrica. 



The organs of vision, like the nervous and muscular sys- 

 tems and all the other organs of relation, present a high degree 

 of development in the lively and active articulated animals, 

 and they are common in almost every class of this division. 

 From the general structure and habits of the entozoa and the 

 density and opacity of the medium in which they commonly 

 reside, they little require the aid of eyes or of any other or- 



