COMPOUND EYES OF ARTHROPODS. 319 



Btructures, as in Homarus or in Penaeus, giving rise to tliu much 

 discussed " spindle." 



But what lias become of the central element wliicli is bo 

 conspicuous in the ommatidium of Limidus, if the retinnlae in 

 all Arthropods are homologous? I believe the central cell is 

 fully functional, judging from its position and from its veritable 

 connection with optic nerve fibre in Limulus. AVhat in other 

 Arthropods strongly reminds one of this cell is the "hyaline cell" 

 at the bottom of tlie ommatidial pit in SeroUs and, according to 

 Beddard, also in the Cyrnothoidae. One important difference, 

 however, exists between the "hyaline cell" of the Isopods and 

 the central cell in the ommatidium o^ Limulus, viz. that, while 

 in the latter the cell is connected with the optic nerve, the 

 "hyaline cell" in the former has no connection with the central 

 nervous system whatever. Hence the " hyaline cell " cannot be 

 sensory, even if it be homologous with the central cell oi Limulus, 

 •which it resembles in its general appearance and in its position. 

 The number of "hyaline cells" in SeroUs is always two, while . 

 its supposed homologue in Limulus is, as a general rule, only 

 one. This fact does not offer any objection to my view of their 

 homology, when wo bear in mind that other elements in different 

 omniatidia, as vitrellae and retinnlae, show a wide range of 

 variation so far as their numbers are concerned, and yet they 

 can be considered as perfectly liomologous. 



A further embryological and comparative knowledge in regard 

 to the "hyaline cell" in Isopods is necessary for the determination 

 of its exact homology. Meanwhile I would observe that if the 

 central and the peripheral cells which we see in the ommatidium 

 of Limulus may be taken as the two essential factors of the 

 sensory element of the typical Arthropod retina, the case of 

 SeroUs may be taken as a loss of balance in the relative develop- 

 ment of these two factors ; the central cells having lost their 

 sensory function and remaining as a sort of supporting niechan- 

 ism. We can imagine this change in the function of the central 

 cell as carried still further, and with the excessive development 

 of the peripheral elements, the retinnlae, the central element 

 may finally have disappeared. 



All this is, however, a mere suggestion, and my interpretation 

 of the nature of the Arthropod ommatidium in general does not 



