SECT, vi THE SENSORY ORGANS 91 



of the cuticle as we find in Limulus projecting inwards 

 became separated from the cuticle, and thereby, 

 naturally, surrounded by their secreting hypodermis 

 cells, and we have at once the Crustacean crystal 

 cones and cone-cells. 1 



In the eye of Limulus we further find visual sensory 

 cells forming groups or retinulae at the tips of the 

 cones, these latter having been gradually pushed 



c 



hy '" 



r - 



FIG. 22. Section through the eye of Limulus (after Lankester). c, cuticle which 

 grows into conical papillae directed inwards, and pushing down the hypodermis 

 cells {hy) ; at the tips of the cones are found the retinulae imbedded in connective 



down below the hypodermis into the subhypodermal 

 connective tissue. We find exactly the same in the 

 eye of Apus (see Fig. 23), where the crystal cones and 

 the hypodermis cells form the original hypodermis 

 layer, the retinulae having been pushed down even 



1 Grenadier in his Sehorgane der Arthropoden states that the cones 

 in Limulus have nothing to do with the Crustacean crystal cones. 

 Our contention here is, however, that some such conicle projections 

 of the cuticle, not necessarily exactly similar to those in Limulus, may 

 easily be supposed to have produced the Crustacean crystal cones, by 

 being separated from their cuticle. 



