Eyes of Molluscs and Arthropods. 673 



of certain Spiders (Grenachek) seem to be formed of three layers; the 

 middle fibrous one probably represents the outer wall of an optic vesicle 

 together witli the connective tissue layer. lu Peripatus all four layers 

 are preseiit PI. 32, fig. 142). In these primitive eyes, it is probable 

 tliat the retinulae were reduced to cover cells, while the retiuophorae 

 alone gave rise to the rods. Judgiug from the strueture of the cells aiid 

 rods as seeu in the simplest larvai ocelli, i. e. Phryganea, it is prol)- 

 able that the retinophorae were very similar to those of the Mollusca 

 and Worms Alciopidae) , i. e. composed of double cells and double rods 

 PL 32. fig. 141) ; from such a simple retina, or retineum, the more mod- 

 ified Arthropod ocelli bave origiuated. The modification was in 

 two directions: (1) an increase in the number of optic ves- 

 icles, accompauied by a reduction in the number of omma- 

 tidia, which at the same time acquired additional size and 

 complexity ; and (2), a decrease in the number of ve si ci es, ac- 

 compauied by an increase in the number and complexity of 

 the ommatidia. That the optic vesicles may varyin number is suffì- 

 ciently shown by the Mollusca and Worms. C banges in the number 

 of eyes are always accompanied by variations in structure. 

 The lateral ocelli of Limulus may be regarded as optic ves- 

 icles whose inner wall has been reduced to an ommateum 

 containing but one ommatidium. A comparison with an ommati- 

 dium of the ancestral eye will show how far the specialization has been 

 carried: one sees that the retinophorae of each ommatidium bave been 

 increased to 10 in number. while upon their axial faces are formed the 

 rods. The changes, then, undergone by the lateral eyes of Limulus 

 were an increase in the number of ocelli, with a conversion of the an- 

 cestral retineum iuto an ommateum, the rods being transferred! 

 from the apices of the retinophorae to their axial faces. The number \ 

 of ommatidia was theii reduced to one, while their retinophorae were 

 increased from two, to ten. In the centrai eye, the reduction in the 

 number of ommatidia has not proceeded so far, there being about 20 ' 

 according to Lankester); neither, as we should expect, is the spec- 

 ialization of the individuai ommatidium so great, since each one pos- 

 sesses ouly six or seveu retinophorae. 



If we seek for some other form, in which the series of changes 

 supposed to bave been undergone by Limulus has been less extensive, | 

 we shall find it in the Scorpions. Here, the lateral eyes bave increased ' 

 but little in number, while there is a correspondingly small decrease in 



Mittheilußgen a. d. Zoolog. Station zu Neapel. Bd. VI. 45 



