COMPOUND EYES OF ARTHROronS. 209 



consisting of seven cells; then come in the outside four vitrellae 

 nrrangecl in a square; then two corneagon cells; and outermost 

 of all, four pigmented cells which completely envelop tlie 

 dioptric part of the ommatidium. 



d. Ilomariis. 

 (Fig. 34, PI. XXXI.) 



The compound eye oi Homarus has quite recently been studied 

 by Parker.' I do not find any facts in Parker's paper which 

 contradict the view maintained in the preceding pages in regard 

 to the nature of the ommatidium. The threefold differentiation 

 of the ectodermal cells in the walls of the ommatidium is just as 

 evident here as in the forms already considered. The problem- 

 atical body, the spindle (Grenadier's rhabdom), like the corres- 

 ponding structure in Camharus, resolves itself into a number of 

 individual retinula cells with thick, serrated, chitinous secre- 

 tions as in the case of Camharus and Penaeus. 



In Fig. 34, PI. XXXI, four retinula cells are i)artly isolated 

 from one another. The lower end of each retinula cell is pro- 

 longed into an optic nerve fibre, while the upper end is devoid 

 of. any pigment granules. A large translucent nucleus exists 

 near the distal extremity of the retinula. The distal end of each 

 cell is prolonged into a short, clearly defined, tapering process. 

 The development of the chitinous serrature is greatest in the 

 middle of the chitin-secreting border of the retinula; when, 

 therefore, the whole is brought together into a bundle, the 

 general outline of such a structure is spindle-shaped. 



e. Callinectes. 

 (Pigs. 5, 5r/, PI. XXIX ; Fig. 37, PI. XXXI ; Fig. 73, PI. XXXV.) 

 Callinectes presents the same interesting modification which 

 we saw in Talorchesiia. We find the corneagen cells and the 

 vitrellae in two successive strata, but no special pigment cells 

 embracing this dioptric mechanism exist. The packing cells 

 ■which we saw in the intervening spaces of the ommatidia in 

 Talarckestia are not found in Callinectes. 



' G. H. Parker : A Preliminary Account of the Development and Ilistology 

 of the Eyes in the Lobster. Proc. Amer. Acad. Vol. XXIV, 18«8. 



