202 



ANDREWS. [Vol. VII. 



It thus appears that in the formation of the eyes of Nereis 

 there is no sign of any invagination. The eye arises as a solid 

 mass in the epidermis. The pigment recedes from the cuticle, 

 leaving a clear mass next it, which subsequently is distinguish- 

 able as rods and lens, and later separates more or less from con- 

 tact with the cuticle. The arrangement of nuclei as seen in 

 section and the appearances seen in macerations strongly sug- 

 gest that the whole eye is merely a collection of epidermal cells 

 with clear ends. 



Proccerea tardigrada. 



The larvae, while still in the brood sac, but capable of swim- 

 ming when liberated, have the form indicated in Fig. 58, and 

 are about . 1 5 mm. long. The eyes are four : two larger ventro- 

 lateral ones very far apart ; two small dorsal, posterior eyes, 

 near together. At an earlier stage all four are nearly in the 

 same transverse line. 



Each is a spheroidal mass of bright yellow-red pigment, near 

 the cuticle, but sending inwards a few lines of granules. In 

 each is a clear, spheroidal, refracting lens (Fig. 57), partly sur- 

 rounded by the pigment granules and in contact with the cuti- 

 cle. In this transverse section of the head there is a solid mass 

 of ectoblast between the cuticle and the "punctsubstanz." In 

 this no cell boundaries are seen, but the arrangement of the 

 nuclei about the eye may be explained if it is made up of a 

 few epidermal cells. Macerations give, as in Fig. 63, a collec- 

 tion of elongated cells closely associated with the pigment and 

 with the lens. These cells have processes at the inner ends, 

 and one is seen connected with a cell, probably a ganglion cell. 

 That the pigment is in the outer ends of these cells, and thence 

 passes down along their surfaces, seems probable from macera- 

 tions in Haller's liquid, osmic acid, acetic acid, and potassium 

 bichromate ; yet this could not be satisfactorily demonstrated. 

 In fact, the last reagent gives results suggesting that the pig- 

 ment is in a special pigment cell sending processes down between 

 the columnar cells, but no nucleus was found in this pigment 

 mass. 



The posterior eyes are as yet very small, and furnish early 

 stages in the formation of the eye. 



The earliest observed is that in which there are seventeen 



