SECT, vi THE SENSORY ORGANS 101 



attention. It is generally referred to as the x-shaped 

 eye-spot which occurs throughout the Entomostraca, 

 but disappears during larval life in the Malacostraca. 

 It is, as a rule, so small, that investigation of its 

 finer structure is difficult. In Apus, however, the 

 unpaired eye is so large that its finer details are made 

 out with comparative ease. This fact is especially 

 important from our point of view, for if Apus is really 

 the (or a) primitive Crustacean, then all the unpaired 

 eyes throughout the whole class are in all probability 

 only modifications of that of Apus. Hence it is 

 necessary, for a comparative study of these organs, to 

 have some accurate knowledge of their original form. 

 We feel justified in assuming that this organ in Apus 

 is in its original form, not only because Apus has 

 retained so many primitive (i.e. Annelidan) character- 

 istics, but because, as will be described below, this 

 form gives us a clue as to the origin of the organ out 

 of an anterior pair of Annelidan eye-spots. In these 

 pages we must of course confine our attention exclu- 

 sively to the unpaired eye of Apus, describing in 

 order (i) its general structure ; (2) its probable origin ; 

 and (3) its present functions. 



Structure. Two groups of sensory cells, each form- 

 ing what is in this connection generally called a retina, 

 yield the two side walls of a cavity which is flat at the 

 top and rounded below. The top consists of the chiti- 

 nous fold, already described as forming the water-sac, 

 while the lower part hangs free in the body cavity. 

 Anteriorly, the cavity runs to a point along the water- 

 canal (sec Fig. 25) ; posteriorly, it ends in a blunt 



