i6o 



ANIMALS 



this is more marked at the anterior end of the body. It is 

 possible that some of the sense organs scattered over the surface 

 of the body are light-sense organs, but there is no direct evi- 

 dence that such is the case. In many other worms, however, 

 there are organs which are unquestionably eyes. They are 

 usually on the head, but may be found else- 

 where, and they vary greatly in number. 



377. One of the simplest of eyes consists of a 

 single epidermal sensory cell, surrounded by a 

 group of pigment cells. More often there are 

 a large number of sensory cells in a compact 

 group. When this is the case the epidermis at 

 this point is greatly thickened, owing to the 

 elongation of cells, and it is also usually con- 

 cave toward the surface. The sensory cells 

 taper below the nucleus into a slender nerve 

 fibre which goes to a deeper lying ganglion. 

 Above the nucleus the cell body is cylindrical, 

 and from its end there project a number of 

 slender bristle-like processes. There is always 

 considerable pigment in such an eye. It lies 

 either within the sensory cells themselves or else 

 in the surrounding non-sensory cells. The 

 cuticula over the eye is usually much thick- 

 ened, and often has the double convex form of 

 a condensing lens. Often, as in nereis, the sensory area sinks 

 in so deeply that it approaches a complete sphere in form. 

 It may then also separate entirely from the epidermis. 



378. In Arthropods the optic apparatus attains a much 

 higher degree of functional perfection, and at the same time 

 becomes much more complex. Its development, however, 

 proceeds along very different lines. An eye similar in structure 

 to that described for worms is found in many Crustacea, and 

 the ocelli of insects are also much the same, but the com- 



FIG. 79. Dia- 

 gram of an om- 

 matidium. a, 

 Cuticular cor- 

 nea; b, corneal 

 cells; c, cone 

 cells; d, retinal 

 cells; e, rhab- 

 dom; /, fibres of 

 the retinal cells. 



