THE SENSORY ORGANS. 



329 



D. The Sensory Organs. 



The Ocelli. 



Detailed accounts have recently been given of the development of 

 the ocelli by Patten (No. 67). There are, on each side, six ocelli 

 which, according to Patten, are distributed in three pairs on what 

 he assumes to be the three most anterior cephalic segments. The 



Fig. 162. — Two stages in the development of the fifth ocellus of an Acilius larva (after 

 Patten), c, cuticular rods ; el, rudiment of the chitinous lens ; h, hypodermis; I, lentigen 

 layer (vitreous body); n, nerve; r, rudiment of the retina; sp, vertical slit in the retina; 

 t, the retinal cells bordering this slit laterally. 



individual ocelli of these three pairs differ considerably from one 

 another in structure and development, although a certain uniformity 

 of type can be recognised. The fifth ocellus (the ventral ocellus of 

 the third pair which, however, has shifted far forward in the larva) 



