THE EYE IN CERTAIN CYMOTHOID^. 447 



It would be an additional merit if they served to render clear the modifications 

 of the eye in a single phylum — the Crustacea — where it can hardly be doubted 

 that the eyes are genetically connected. 



Dr Patten states (p. 677) that " the presence of the corneal facets in certain 

 higher forms only of insects and Crustacea indicates that they are of late 

 origin ; moreover, the presence of a thick corneal hypodermis and the absence 

 of corneal facets in such animals as Branchipus, the Isopods, Amphipods, and 

 many insects show this condition to be a primitive one." Turning to the 

 account of the structure of the eye in the amphipoda given in Bronn's " Thier- 

 reichs " (p. 343), I find that this very character, viz., the presence or absence of 

 corneal facets, is made use of as a mark of distinction between the two groups of 

 the Amphipoda and Isopoda — " Die structur der Amphipoden-Augen anlangend, 

 so weichen sie von denjenigen der Isopoden dadurch sehr wesentlich ab, dass 

 der sie bekleidende Theil des Kopf-Integumentes in keine nahere Beziehung zu 

 den lichtbrechenden Medien tritt, dass mit anderen Worten also Cornea-Bil- 

 dungen vollstandig fehlen." It had not occurred to me, even before referring to 

 the literature* of the subject, that in figuring the corneal facets in the Cymo- 

 thoidse, I was adding a new fact to our knowledge of Isopod anatomy. Dr 

 Patten has also announced the discovery of the cuticular hypodermis, which, in 

 all the Arthropod eyes examined by him, was found to intervene between the 

 cuticle and the cells which secrete the vitreous bodies ; it is true that these 

 structures have been for the most part overlooked, but I would refer Dr 

 Patten to fig. 2 of plate xlii. of the same volume, where they are distinctly 

 figured in an Amphipod (Phronima sedentaria).\ I have not succeeded in 

 finding them in any of the species of Cymothoidse which I have examined, but 

 have not the least doubt that Dr Patten is right in supposing that they exist 

 in all compound Crustacean eyes. 



Whatever may be the value of Dr Patten's statements respecting the 

 continuity of the crystalline cones and the rhabdom in the Decapod eye, there 

 can be no question that at least in many Isopoda these structures are perfectly 

 discontinuous. Grenacher's figure of Porcellio shows this fact, and my own 

 figures illustrating the present memoir are in complete accord with Grenacher's. 

 I have not been able to trace any continuity whatever between the rhabdom 

 and the vitreous body, and the interval between these two structures is (see 

 figs. 1, 2, &c.) a considerable one. Moreover, the cells which secrete the 

 crystalline cones are bounded by a very distinct outer layer, which is as obvious 

 at the lower end as at the upper end of these cells (see fig. 1). Hence it is 

 clear that in these Isopods, at any rate, the cells of the vitrella are not prolonged 



* Cf. also Bullar's paper quoted above (pi. 46, fig. 12). 



f Mentioned also in Balfour's Comparative Embryology, vol. ii. p. 396 ; see also Caeeieee, Sehorgan 

 der Thiere, 1885, p. 158, where they are figured and described in Gammarus jpulex. 



