ESTERLY : EUCALANUS. 39 



and nerves. The relation of the latter to the cells is not the same as in 

 the median eye, however. For there we have seen that, whether the 

 "central cell" (pigment cell) or the basal plates are regarded as the 

 pigment-bearing portions of the eye, the nerves leave the retinal cells 

 from their basal ends, or that portion of the cell adjoining the pigmented 

 part of the eye, whereas in an organ of Claus, the nerve leaves that 

 portion of the periphery of the cell which is farthest removed from the 

 basal plate (Plate 2, "Figs. 19, 20; Plate 3, Figs. 26, 29; Plate 5, 

 Fig. 55). Consequently, if we follow consistently the interpretation 

 hitherto given of the relation between a sensory cell of an optic organ 

 and its nerve fibre, the organs of Claus are to be regarded as hi-ceUular, 

 inverted ej/es. In other words, the ends of the nerve fibrils are directed 

 toward the bases of the sensory cells. But in the median eye, the 

 light recipient portions of the optic nerves are directed toward the 

 outer ends of the cells. 



III. Discussion. 



From the previous references to the literature bearing upon the 

 subject, it has appeared that, with the exception of Grenacher ('79), 

 all who have studied the median eye of Crustacea have either defi- 

 nitely stated their belief in the inverted character of the retinal cells 

 (Hesse, ;01, :02; Claus, '91, Hartog, '88; Richard, '91), or, taking 

 neutral ground upon this particular point, have felt that a close com- 

 parison with the eyes of the flatworms was justifiable (Carriere, '85; 

 Lang, '88 94; Claus, '63). A quotation from Hesse (:02, p. 630) will 

 put this matter concisely, and since it comes from one whose knowledge 

 of the optic organs in invertebrates is unexcelled, it may be considered 

 as representative. "Die invertirten Pigmentbecherocellen haben 

 cine sehr weite Verbreitung. Alle Sehorgane, die wir bei den Plathel- 

 minthcn kennen, sind hierher zu zahlen: also die Ocellen der Tur- 

 bellarien, die x-formigen Augenflecke der Trematodenlarven, und 

 die Ocellen der ausgebildeten ektoparasitischen Trematoden, die 

 Ocellen der Xemertinen und wahrscheinlich audi diejenigen der 

 Rotatorien. Ferner gehoren hierher mit grosser Wahrscheinlichkeit 

 die Ocellen der Trochophoralarven und jUmlicher Larvenformen, 

 sichcr die Ocellen des Nauplius und die mit ihnen identischen Med- 

 ianaugen vieler ausgebildeter Crustaceen. . . ." From this point of 

 view it may readily be seen how it is possible for such a statement as 

 the following (Hesse, :02, p. 647) to be made: "Bei den Crustaceen 



