60 DISCOPHORiE. Part III. 



very large, in the young, are a kind of flat tentacles, now hardly more projecting 

 than the lobules between the adjoining tentacles. 



In view of a proj^er appreciation of the morphology of the Acalephs, it is 

 important to bear in mind that all the marginal appendages of these animals, 

 whether solid or hollow, whether in the direct prolongation of the radiating tubes, 

 or arising from the circular tube, bear the same relations to the aquiferous system. 

 They are everywhere implanted upon its outer edge, and when hollow, are in direct 

 communication with it. This is the case of the tentacles proper (PI. IX. Fig. 3 

 d' d'), as it is, also, with the lappets of the eye (e i' i') and with the eye proper 

 (o). Compare, also, PI. VI. Fig. 4 and PI. XP. Fig. 17 and XP. Fig. 11. And if 

 we take into consideration the fact that there is no essential difference between 

 the tentacles at the base of which there is no accumulation of pigment, and those 

 in which pigment accumulates to such a degree as to assume the appearance of 

 an eye-speck, and further that we have well-developed eye-specks at the base of 

 equally well-develojjed tentacles, we shall not be inclined to consider as essentially 

 different, these organs in which the tentacular element is reduced to a minimimi, or 

 entirely wanting, and the ocular element developed to a maximum degree of special- 

 ization, as is the case in the eye of Aurelia wdtli its peduncle, hollow as a tentacle, 

 and its lobules projecting like tentacles. But, however perfect and eye-like the 

 visual apparatus of these animals may appear, it must be remembered that, in its 

 morphological relations, it is a dependence of the system of radiating tubes, and can 

 in no way be homologized Avith the eyes of animals belonging to other branches 

 of the animal kingdom, in which the organs of sight are formed in a totally different 

 way. I hold that in all Radiates, from the Echinoderms to the Polyps, the marginal 

 pigmented appendages of the aquiferous system are homologous to one another, and 

 that, by their function, they are visual organs, even though they are not eyes, as 

 we find them in other types. 



The gelatinous disk, at first regularly lenticular, with a uniformly convex outer 

 surface and a uniformly concave inner surface, thickest in the centre and gradu- 

 ally thinning out to the margin, remains uniform on the outside, and the only 

 change which its upper surface presents, consists in an increased unevenness, arising 

 from the crowding of epithelial and lasso-cells, which form little inequalities on the 

 surface, as represented PI. VIII. Fig. 4. The inequalities which are gradually form- 

 ing on the lower surface of the ujiper gelatinous flooi', and which consist chiefly 

 in the rising of a central eminence projecting into the main digestive cavity, and 

 four radiating keels intercepting the four genital pouches, have already been described. 

 But a profile view of our Aurelia, such as PI. VIII. Fig. 1 represents, exhibits 

 these inequalities most distinctly, and especially the encroachment of the sexual 

 pouches into the substance of the disk, and shows further, very plainly, how the 



