416 



GENERAL CONSIDERATIONS ON 



depressions, which, however, close to form vesicles and become 



separated from the hypodermis. 



3. 



The lens is secreted within the 

 optic vesicle. The 

 eyes of Peripatus 

 thus, in their onto- 

 geny, pass through 

 the stage of the sim- 

 plest Arthropod 

 eye, but then rise 

 to a higher level 

 than that attained 

 by the latter, and 

 can far better be 

 compared with the 

 higher forms of 

 eye found in the 

 Annelida. In any 

 case, we do not 

 recognise in them 

 the Arthropodan 

 type of eye. 



The facet -eyes 

 of the Insecta 

 must be regarded 

 as arising from a 



I- 



massing 



together 



of simple ocelli in 

 the way already 

 indicated in the 

 Myriopoda. The 

 latter group, in 

 the simplest cases, 

 have only a few 

 ocelli on each 

 side (Scolopendra 

 four), but their 

 number may in- 

 crease (Lithobius, 

 Julus, thirty to 

 forty on each side), and in some forms (Smtigera) there may even be 

 as many as 200 ocelli on each side, which, by their close approxima- 



te. K»7. — A-D, diagrams illustrating the development of an 

 ommatidium from a depression of the hypodermis. D represents 

 an ommatidium from the compound eye of an Amphipod (Talor- 

 chestia, after Watase). c, central cell; ch, chitinous covering 

 of the head ; h, hypodermis ; k, crystalline cone ; !>:, crystalline 

 cone-cells; I, lens; lg, lentigen cells; n, nerve; rli, rhabdom ; 

 rt, retinula cells. 



