Miscellaneous. 239 



or, most frequently, concentrated by a convergent lens. In all cases 

 the image produced is reversed. 



The second kind, misunderstood or neglected by many anatomists, 

 consist essentially of a blackish or reddish pigment, of very definite 

 structure, impressionable by luminous rays, and in immediate relation 

 to the nervous system in animals which are provided with the latter. 

 The sensitive pigment does not necessarily occupy a point on the 

 outer surface of the body ; the very simple functions assigned to it 

 may be performed when the pigment is separated from the external 

 medium by translucent bodies — such as the exterior tegument, for 

 example. This is the arrangement that I have met with in the Si- 

 punculidae ; it has also been indicated in certain Annelides, especially 

 the Hermellop, in which its nature has been well comprehended by 

 M. de Quatrefages. 



Thus constituted — that is to say, reduced to an aggregation of pig- 

 ment-cells in connexion with the nervous system, or, still lower in 

 the scale, only with the sarcodic tissue, and receiving the impression 

 of the luminous rays mediately or directly — they represent the 

 simplest form of the organ of vision in the animal series. It is in 

 this simple state that they have been described by Rathke in the 

 genus Lycoj'is, and that they are met with in many other genera. 



In examining recently the composition of the pigment-spots (well 

 known to naturalists) which occupy the extremities of the rays of 

 Aster acanthion ruhens, we have discovered an organic perfection of 

 the photoscopic eye which appears to have hitherto escaped the 

 notice of observers. 



The pigmentary eyes of Aster acanthion are situated at some little 

 distance from the extreme end of the rays, in the interambulacral 

 furrows. They occupy a small papilla or gemmiform tubercle, 

 which receives a filament from the ambulacral nervous trunks ; and 

 this filament becomes dilated into a ganglion on penetrating into the 

 papilla. The spiniform calcareous processes which terminate the 

 arms of the Starfish surround the papilla like a sort of calyx, which, 

 however, is open at the level of the interambulacral furrow. When, 

 by the action of the muscles, these processes are separated from each 

 other, the visual organ is completely exposed, and receives the lumi- 

 nous rays without any obstacles. When, by the contraction of the 

 antagonistic muscles, these processes are approximated and brought 

 into contact, the calyx closes and imprisons the oculiferous papilla ; 

 the luminous rays can then no longer reach it, and, if I may use the 

 expression, the eyes are shut. The Starfish can therefore, at its 

 pleasure, exercise or suspend the act of vision, and effectually pro- 

 tect the organ of sight from the injurious contact of external objects. 



When the oculiferous papilla is examined under the microscope, 

 the presence of the red pigment which we have already mentioned 

 is easily detected ; and it may also be remarked that this pigment 

 does not cover the surface of the papilla with a uniform layer, but 

 that it is distributed upon it in clearly defined and regularly distri- 

 buted groups. If, with the view of ascertaining precisely the nature 

 of these pigment-groups, a magnifying power of 300 or 400 dia- 

 meters be employed, the oculiferous papilla is seen to be hollowed 



