104 



THE PINEAL ORGAN 



eye itself is composed of a large number of radially arranged units called 

 ommatidia, each of which consists of an outer and an inner segment. The 

 outer segment is formed of clear cells enclosing within them a conical 

 vitreous body, the crystal cone. The inner segment is formed by a group of 

 sensory cells enclosing a clear axial rod, the rhabdite and forming with the 

 former a retinula. The retinula is the actual sensory part of the ommati- 

 dium, and its cells are comparable in this respect, although not in details 

 of structure, with the sensory cells of the retina in vertebrates. The 

 retinula; of adjacent ommatidia are separated from each other by a 

 circumferential zone of cells, containing black pigment. 



Fig. 67. — Dorsal Aspect of Apus Cancriformis, a Small Fresh- water Crus- 



tacean of Primitive Type, 

 Compound Lateral Eyes. 



abd. : abdomen. 



af. : caudal style. 



d.o. : dorsal organ. 



possessing a Single Median Eye and Paired 



E. : paired lateral eye. 

 e. : median eye. 



th.f.l. : endites of first thoracic 

 foot. 



(From Bronn's Thierreich.) 



The sensory cells of the retinulae appear to be continuous with corres- 

 ponding separate fibres of the optic nerve and since each ommatidium is 

 surrounded by pigment cells and is thus isolated from its fellows, it 

 would be able to transmit one part only of the whole field of vision to 

 the particular nerve fibres which are directly connected with its sensory 

 cells. The development of the median eye of Apus is similar to that of 

 Cyclops, in which a deeply pigmented and bilobed eye-spot is developed 

 (see description by Patten alluded to in Chap. 24, p. 360). 



In Cypris the nauplius or larval form of Lepas fascicularis (allied to 

 Lepas anatifera, the common barnacle) (Fig. 70), both median and 



