C40 Edgar Davidson Cougdoti 



II PALAEMONETES VULGARIS STIMP. 



The eyes of the prawn consist of a stalk and terminal bulb. 

 The former contains a series of four optic ganglia, which are 

 enlargements of the optic nerve. The bulb is made of numerous 

 rod-like ommatidia, which extend somewhat radially from a base- 

 ment membrane (Fig. i, mb. ba.) near the end of the stalk to square 

 facets {eta.) in the cuticula covering the bulb. An ommatidium 

 may be roughly divided into a distal two-thirds, consisting chiefly 

 of cone cells, and a proximal third containing the rhabdom. The 

 four cone cells {cl. con.) lie parallel to the axis of the ommatidium, 

 and in their distal portions are closely associated to form the glassy 

 cone. They taper proximally from the cone to the rhabdom 

 whence they possibly extend as processes to the basement mem- 

 brane. Between the cone and the facet are two small corneal 

 hypodermal cells {cl. cm.) Six distal retinular cells full of black 

 pigment together form a sheath around the cone cells (Fig. 3). 

 They do not belong exclusively to one ommatidium, but each 

 one serves as a partial sheath for three cones. The lower third 

 of the ommatidium contains seven proximal retinular cells (Fig. 

 I, cl. /?x.), lying close together and parallel to the ommatidial 

 axis; they extend as long processes into the region proximal to 

 the basement membrane. Distal to this membrane they unite to 

 form the spindle shaped rhabdom {rhb.) They contain black pig- 

 ment. Associated with them are one or two whitish accessory 

 pigment cells. Fibrillae from the distal optic ganglion extend up 

 through the proximal retinular cells to end in the rhabdom. 

 Distally, the central parts of the ommatidium are transparent and 

 convey the light to the rhabdom, which is thus open to stimulation. 



The photomechanical changes of Palaemonetes have been de- 

 scribed by Parker ('97). In increasing light the distal retinular 

 cells migrate as wholes in a proximal direction, thus restricting, 

 as Exner ('91) has pointed out, the amount of light that enters the 

 deeper parts of the eye. The pigment of the proximal retinular 

 cells is at the same time carried distally along the sides of the 

 rhabdom, probably by protoplasmic streaming within these cells. 

 This process also reduces the amount of light that can reach the 



