700 William Patten 



In Aurelia, according- to Hertwig, the ocelli are very poorly de- 

 fined, and are not provided with auy special thickening of the cuticula. 

 They probably represent only the simplest aggregation of ommatidia. 



It is very probable that the so-called lens of the simple, non-in- 

 vaginate ocelli is not a lens at ali, but a lenticular thickening of the 

 cuticula, filled with nerve fibres; or, in other words, it is composed 

 of the cuticular secretions, or rods, of the ommatidial cells, and con- 

 tains the retiuidia. Such a condition could be compared directly with 

 the pseudo-lenticulate eyes of^;ra(Pl. 30,fig. 54). It is extremely 

 improbable that a purely di op tri e lens would be developed be- 

 fore the specialization of the rods. Another striking faet that deserves 

 attention is the wide presence of red, and less often, black pigment, 

 which when united give rise to shades of brown varying in color ac- 

 cording as one or the other pigment prevails. 



Another important point to be considered is the great number 

 of imperfect eyes, and the presence of very highly developed ones, 

 as in Charyhdaea. This condition is exactly parallel to that found in 

 Arca, and brings us a confirmation of our supposition that the evaginate 

 eyes of ^rm did not arise independently, but as modifications of the 

 invaginate ones. If, for instance, both types of eyes were developed from 

 an irregularly folded surface, thehoUows forming into invaginate eyes, 

 and the ridges into the evaginate ones, then we should expect to find as 

 many transitional stages in the development of one form, as in that of 

 the other, especially when there were a great many of both kinds in 

 ali stages of development. This, as we bave seen in Arca, was not the 

 case. In Coelenterates also, although there are many very simple in- 

 vaginate eyes in ali stages of development, there are no traces of the 

 evaginate ones. 



In the Worms, that uniformity in the structure of the sensitive 

 layer, found in Mollusca and Coelenterates, does not appear to prevali. 

 The eyes found in the Annelids are best understood; therefore they 

 cau be more readily compared with those found in the Mollusca and 

 Arthropods. We shall speak of them alone. 



Carrière has furnished us with the best observations concerning 

 the eyes of Polychaetous Annelids. The same composition of the sensi- 

 tive layer, and of the ommatidia, is to be seen bere as in Mollusca. The 

 colorless cells (Secretzellen of Carrière) are the retinophorae, and the 

 pigmented ones, the retinulae. It is remarkable how a person can fall to 

 be impressed with the similarity between the colorless cells of the Coe- 

 lenterate retina, and similar ones in the retina of Molluscs. orWorms. It 



