32 bulletin: museum of comparative zoology. 



"possibly in Copepods." The essential point is that the retina is 

 separated from the hypodermis, but not by the formation of an optic 

 pocket. The method of separation in Amphipotls is by means of a 

 membrane, the corneo-conal membrane, which is, in Parker's opinion, 

 composed of two layers, one formed by the retina, the other by the 

 hj^odermis. The two portions of the corneo-conal membrane are 

 seen to be separate at the edge of the retina, one being the basement 

 membrane of the hypodermis, the other forming the capsular mem- 

 brane, which envelopes the retina and is finally reflected over the optic 

 nerve. In Gammarus the corneo-conal and capsular membrane 

 completely enclose the retina and separate it from all other tissues 

 with the exception of the optic nerve. 



In Copepoda, too, the retina is separate from the hv|3odermis, and 

 in Argulus the separation is made more extensive by an intervening 

 blood space. The retina in the Eucopepoda as represented by the 

 Pontellidae and Corycaeidae is apparently not continuous with the 

 hypodermis, but Parker ('91, p. 59) states that it is difficult to decide 

 to which of the three types the retina in Copepoda belongs. ". . . . If 

 the lateral eyes in Copepods are not representatives of a fourth type, 

 essentially different from the three already described, they must be 

 considered members of the third retinal type." Hesse (:02) has 

 grouped the eyes of many invertebrates in a way similar to that em- 

 ployed by Parker ('91), but the work of the former writer will be 

 referred to more extensively later. 



It may perhaps be questioned whether the median or "nauplius" 

 eye of such Copepoda as Eucalanus, Cyclops or Diaptomus, may be 

 justly com])ared Avith the eyes of a distinctly higher type, which are 

 found in other Crustacea. But the relation of the parts of the eye to 

 the ectoderm in Eucalanus are striking, and exactly along the lines 

 marked out by Parker ('91) in his treatment of the compound eyes 

 in Crustaceans. It seems to me, therefore, that it is worth while to 

 consider the matter, since it is important in a discussion of the phy- 

 logeny of the median eye. 



Nothing is known concerning the ultimate relation of the median 

 eye to the hvpodermis, or other membranes, except that it is developed 

 from ectoderm (Grobben, '81; Urbanowicz, '81; Claus, '91, p. 259), 

 as in all other Crustacea. Claus ('91. p. 200) treats of the enveloping 

 membranes in the following words : " Endlich hat der mehr oder minder 

 herabgeriickte Augenbecher, und im Falle der Vereinigimg der drei- 

 theilige Augencomplex, eine mesodermale Umhlillung erhalten, welche 

 sich direct in das Neurilemm des zur Retina tretenden Nerven 



