252 ORGANS OF THE SENSES. 



choroidal pigment the eyes of the rotifera, like those of the 

 epizoa and those of most of the higher articulata and mol- 

 lusca, are very obvious in the embryos while yet within the 

 ovarium, or while the unhatched ova are yet suspended from 

 the exterior of the parent. They are commonly placed near 

 the large supra-oesophageal ganglion, and nervous filaments 

 are sometimes observed distinctly to pass into these organs. 

 They are protected by a smooth transparent and sometimes 

 glistening cornea, and when these little eyes are pressed be- 

 tween plates of mica or glass, the eye-ball is seen to burst, 

 and the deep red pigment, consisting of minute globules 

 united together by a transparent matter, is seen to escape 

 from the ruptured organ. A minute crystalline lens has 

 also been observed in some, behind the transparent cornea. 

 The fixed condition of the cirrhopods in their adult state, 

 inverted in their shell, and the opacity of their exterior 

 covering, afford little occasion for the employment of visual 

 organs in that condition of their life, and they have not been 

 observed in any of the mature animals of this class. Before 

 their metamorphosis, however, in their young and free con- 

 dition, the cirrhopods possess a distinct black coloured me- 

 dian organ of vision formed by the juxta-position of the two 

 lateral eyes as in many monoculous animals of other classes. 

 These two eyes, formed by the division of one primary organ, 

 appear to be covered with a smooth transparent cornea and 

 to enclose a small crystalline lens, and they are lost by the 

 metamorphosis like the eyes of many other articulated 

 animals. 



In most of the free annelides there are numerous and very 

 distinct organs of vision, which commonly project shining or 

 glistening from the dorsal surface of the head, and contain a 

 minute transparent lens added to the variously coloured dark 

 pigment and the optic nerve. The eyes are here generally sim- 

 ple, sessile, slightly moveable, and placed apart from each in 

 transverse or longitudinal rows. In the little prostoma arma- 

 tum the head is covered with oculiform points of a dark colour, 

 in the minute prostoma clepsinoideum, a small planaria, there 

 are six of these organs, and in other species of prostoma four 

 are observed. Several of the planaria have two deep co- 

 loured eyes, in some as the planaria viganensis, there is a 

 single row of about forty eyes, passing round all the fore 



