ORGANS OF THE SENSES. 249 



or their polypi from the influence of this agent though desti- 

 tute of visual organs. 



Distinct organs, however, appropriated to light, are already 

 perceptible in animals where no nervous filament has been 

 detected in any part of the body. Many polygastric animal- 

 cules, as the cercaria, are obviously sensible to light, and 

 move towards it like the hydrse, and they generally present 

 on the anterior part of their body one or more small round 

 opaque red coloured spots which have long been recognised, 

 figured, and described as the eyes of these minute animals. 

 Several, as the euglena longicanda and ophryoglena flavicans, 

 have but one eye, placed on the middle of the upper and 

 anterior part of their body, a monoculous character in which 

 they resemble the microscopic species of some higher classes 

 of animals. Eyes have been detected in most of the genera 

 of polygastrica, down to the monads which are mostly mono- 

 culous, and even the minute monad-like beings which unite 

 to form the volvox globator, the eudorina elegans, and some 

 other remarkable compound or aggregate animalcules, are 

 provided with single red-coloured organs of vision placed 

 near the part of their trunk from which the caudiform vibra- 

 tile appendix is prolonged. These minute red points of the 

 polygastrica are as obviously eyes, though of the simplest 

 structure, as are those of rotifera where we can perceive their 

 optic nerves and ganglia, and where they have the same red 

 colour and general disposition as in the polygastrica. This 

 superficial or cutaneous opaque spot, to absorb the rays of 

 light, without forming an image of external objects, is the 

 first form of the eye which we see also in annelides and in 

 the larvae of insects, and the young of higher classes, when 

 that organ is beginning to develop ; so that when the optic 

 nerve is superadded to this pigment, or choroid, or coloured 

 rete mucosum, it is first developed behind this opaque mat- 

 ter, as we frequently find it placed in the eyes of inverte- 

 brated animals, even when transparent parts are added to 

 collect light, or to form an image of external objects to be 

 transmitted through a hole in this choroidal pigment to the 

 subjacent nerve. The numerous vibratile organs of locomo- 

 tion, and the rapid movements and free condition of polygas- 

 trica require this general development of visual organs in 

 them, while the slow movements or the fixed condition of 



