SEA-FISHERIES LABORATORY. 449 



cells surrounded by two pigment cells.) Between the 

 vitrellae and the pigment cells there are intercellular 

 spaces, which are larger at the proximal, end of the 

 vitrellae. 



The retinulae (ret.) are seven pigment cells which 

 surround the rhabdome, and extend from the proximal 

 portions of the vitrellae to the basement membrane. 

 Distally each retinula ends in a rounded knob, which 

 contains a nucleus (fig. 71 shows the disposition of the 

 retinulae around the rhabdome). At their distal ends 

 the retinulae are extremely large, and surround the 

 rhabdome in this region so as to completely hide ix. The 

 concentration of the pigment in this region gives rise to a 

 well-marked pigment band (fig. 69, pg. i.). Proximally 

 the retinulae are not so large, and the rhabdome can be 

 seen quite distinctly between them. The rhabdome has a 

 peculiarly striated appearance, which is caused by the 

 arrangement of the pigment granules of the retinulae. 

 The optic nerve fibres pass into the retinular cells, so that 

 it is this part of the eye which is sensitive to light. 



The rhabdome (rhab.) is a rod-like structure in the 

 centre of the retinulae. According to Watase,* the 

 rhabdome is a chitinous structure produced as a secretion 

 from the retinulae. Parkert states that transverse 

 sections show that the rhabdome is composed of four 

 parallel rods, and he further affirms that these four rods 

 are the inner extremities of the four vitrellae (cone cells). 

 The optic nerve fibres perforate the basement 

 membrane and end in the retinulae. 



Each ommatidium, therefore, may be said to consist 



* Watase, S. "On the Morphology of the Compound Eyes of 

 Arthropods," Q. J. M. S., Vol. XXXI, p. 143. 



f Parker, G. H. " The Histology and Development of the Eye in 

 the Lohster." Bull. Mus. Comp. Zool. Haw., Vol. XX, No. 1. 



"The Compound Eves in Crustaceans," loc. cit., Vol, XXI, 



No. 2. 



