EYES OF CRUSTACEANS 



109 



compound retina of the lateral eyes of vertebrates, which also consist 

 essentially of three layers of sensory elements : 



1. Receptor cells .... Rod- and cone-cells. 



2. Secondary neurones . . . Bipolar cells. 



3. Tertiary neurones . . . Ganglion cells. 



There is this difference, however, between the lateral invertebrate eyes 

 under consideration and the lateral eyes of vertebrates, namely, the 



Fig. 71. — Dorsal View of Cyclops, a Free-swimming Eucopepod (Class 

 Crustacea), showing a Single Median Eye, formed by Fusion of a Pair 

 of Simple Median Eyes. 



abd. : first abdominal segment. ov. : 



an. : anus. th.~ 



ant} : antennule. th. 6 

 e. : median eye. 



ovary. 



thoracic segments, 

 thoracic segments. 



retina of the invertebrate eye is upright, whereas the vertebrate retina is 

 inverted. 



Between the receptor cells and the secondary neurones in both verte- 

 brate and many invertebrate retinae is a plexiform layer, formed by the 

 communicating axonal and dendritic branches of these cells, and another 

 plexiform layer is formed between the axonal branches of the bipolar 

 neurones and the dendritic branches of the ganglion cells. Also in 

 invertebrates, as in vertebrates, the fibres of the optic nerve are carried 



