SECT. XII RELATION OF APUS TO CRUSTACEA 203 



retinulated compound eyes and the single ocelli of 

 Limulus. We find, however, that the ocelli of 

 Limulus, according to Lankester's and Bourne's 

 figures, are only ocelli in the sense that they have 

 but one large cuticular lens ; the retinal cells under 

 them being grouped in retinulae. The presence of 

 retinuLx essentially of the same shape as those 

 under the conical cuticular projections in the paired 

 eyes (see Fig. 22) suggests that this large cuticular 

 lens has arisen by the coalescence of a number of 

 such crystal cones ; otherwise, according to our 

 view, it would be difficult to account for the reti- 

 nulae, which we think first arise by the grouping of 

 the sensory visual cells round the tips of the conical 

 refractive processes. If this is the case, the ocellus 

 of Limulus is not due to an independent utilisation 

 of a special form of cuticular thickening, as we 

 think is the case in such an eye as that of the 

 Dytiscus larva, but, as stated, to a coalescence of 

 the separate crystal cones to form one large lens. 

 The original compound eyes with their separate 

 cones probably formed weak spots in the anterior 

 shield, and therefore gradually developed large single 

 lenses by the concrescence of the cones. One con- 

 sequence of the change is, according to our theory, 

 clear, and that is that the retinute, being no 

 longer grouped round crystal cones, are, as reti- 

 nulcs, comparatively useless. We turned, therefore, 

 with great interest to Lankester and Bourne's 

 account of these retinulse, and found what we 

 expected, that they are by no means so definite as 



