RELATION OF MEDIAN TO LATERAL EYES 361 



opening of the smaller pit in the walls of which the retinal placodes of the 

 median eye are imbedded also becomes closed, only a small recess in the 

 floor of the larger common chamber being left as an indication of the 

 site of the original communication (Fig. 247). The median or parietal eye 

 of Apus, according to Patten's description, forms a closed chamber with 

 a retinal placode on each side wall, and two unpaired placodes one on its 

 posterior and the other on its inner wall. Each placode consists of a 

 single row of large colourless, columnar cells. Their distal ends are 

 buried in a dense mass of dark brown or black pigment, their proximal 

 ends are colourless. As in Branchipus, there are two large cells which 



fe^r- rec.c 



-n.fronb 



n.opt. 



Fig. 249. — Triple Eye of Calanella mediterranea, a Free-swimming Copepod 

 Crustacean, tyjwt, : from below. (After Grenacher.) 



n. front : frontal nerve. p. : pigment plates of the paired eyes. 



n. opt. : optic nerve. ret. c. : retinal cells. 



p. 1 : pigment plate of unpaired portion. 



appear to give rise to the greater part of the pigment that fills the cavity 

 of the vesicle. When the pigment is partially dissolved it is seen that 

 each retinal cell is capped with a large brush-like mass of fine fibres 

 (retinidium). 



Before commencing the detailed comparison of the median with the 

 lateral eyes of arthropods it will be necessary to define the different types 

 of eye which are met with in this phylum, which includes the Crustacea, 

 Insecta, and Arachnoidea. Patten subdivided the eyes of Arthropoda 

 into four types, namely : 



1. Paired larval ocelli. 3. Frontal ocelli or stemmata. 



2. Parietal eyes. 4. Lateral or compound eyes. 



Other authors combine Patten's second and third types into a single 



