LATEEAL AND CENTRAL EYES OF SCORPIO AND LTMULUS. 207 



plasm of these cells with the madder-brown colour formed by 

 the dissolved pigment. 



The rhabdoms of the retinulse are very irregularly developed. 

 Those nearer the centre show, however, under favorable cir- 

 cumstances in transverse section, a five-fluted or seven-fluted 

 rhabdom, as represented in figs. 29, 30. 



It is difficult to estimate the number of the retinulae present 

 in a central eye of Limulus, on account of the want of definite 

 segregation of the nerve-end cells at the periphery of the omma- 

 teum. But indications of about twenty are seen when a com- 

 plete horizontal section is studied. 



The most remarkable feature in the central eye of Limulus is 

 the great development of intrusive connective tissue in 

 front of and around the retinulae. Much of this connective 

 tissue is pigmentiferous, but we must suppose that certain 

 tracts of it in front of the retiuulse are free from pigment, and 

 leave a path for the rays of light. Nevertheless we are not 

 able to say with any certainty which cells are pigmentiferous 

 and which cells are transparent. The large vesicular cells in 

 front of the retinulae, marked s s, are probably free from 

 pigment. 



The branched and fusiform cells, which are seen everywhere 

 in sections creeping over and clothing the retinulae {sz),ave 

 undoubtedly pigmentiferous. 



Comparison or Central Eye of Limulus and Scorpions. 



The great mass of connective tissue present in the ommateum 

 of the central eye of Limulus is to be regarded as a develop- 

 ment of the intrusive connective tissue which we have already 

 seen in the central eye of Scorpions. It is so largely developed in 

 Limulus as to lead one to regard the retinulae as sunk and 

 buried in it, and suggests the possibility that, at any rate in 

 the adult, the central eye of the King Crab may have partially 

 lost its function. At any rate, the irregularly constituted re- 

 tinulae and the abundant connective tissue of the central eye 

 contrast markedly with the cleanly cut retinulae and simple 

 perineural cells of the lateral eyes. 



