300 s. ir-r/'jA'A". 



Tlie tliird stratum of cells, the retimilac, are extroincly 

 eloiignteil, each retiiiuhi with a clear, straiglit, finely striated 

 rhalKhiiiiere. A soinowhat diagraiiiatic representation of the 

 individual rctinula cell with its chitinous harder is j^iven in 

 Fig. 37, PI. XXXI. In Fig. 5a, PI. XX IX, the upper portions 

 of the two retinula cells are shown; tlic rhabdoinere just beneath 

 the crystalline cone shows a slight enlargement. The size of the 

 retinula cell at the distal e.xtreinity undergoes an enlargement. 

 The enlarged extremities of the retinulae form a pigmented 

 collar around the inner end of the crystalline cone. If the 

 upward extension of this part of the retinula cells be carried 

 still further, a condition which prevails in a retinula like that of 

 Talorchedia will be found. In the freshly teased preparation, 

 one or more refractive globules, ranging from yellow to reddish 

 brown in color, are found in the thickened portion of the 

 retinula. The general shape of the ommutidium as determined 

 by the shape of the corneal facet is hexagonal. 



For the general discussion of the subject I do not find it 

 necessary to go into further details of comparison, nor to enume- 

 rate more examples from diflerent Arthropods. In the forms I 

 have studied in this connection, representing near thirty Arthro- 

 pod genera, I found nothing which invalidated the general inter- 

 pretation of the ommatidial structure of the Arthropod as 

 explained in the preceding pages, and the compound eye of the 

 Arthropod, whether it be sessile or stalked, must be held to 

 represent morphologically a group of ectodermal pits or a 

 bundle of ectodermal tubules. 



III. — The Compound (Latkral) Eye of Limui.os. 



The question which next suggests itself is this : Adopting for 

 a moment the view that the ommatidium of the compound eye 

 is morphologically an ectodermic pit, is there any adult arthro- 

 pod whose eye permanently remains in this condition ? 



I find such an ommatidium in the compound eye oi Limulus. 



On this account I will describe the eyes of Limulus somewhat 

 in detail.' In the succeeding section of the paper I will point 



' An outline of this portion of the paper will be found in the March number 

 of the University Circular, 1889, Johns Hopkins University, under the title 

 The Structure and Development of the Eyes of the Limulus. 



