ß72 William Patten 



seems to me, treat those kincl enougli to respond to the call, with cou- 

 sideration, forpersons treated otherwise are liable totake offence, and 

 may find more mistakes than was anticipated. 



There seems to be very little to clioose betweeu the »flüchtige 

 Streifzüge« ofGRABER, and the «eingehenderen und genaueren 

 Studien« of Grenacher. The former has helped outhis observatious 

 with too much theory, the latter has helped his theory with too 

 little Observation. Now since the doctors disagree, it is only left us 

 to choose our own course, picking out, when possible, those facts 

 which seem to poiut in the right direction. Therefore, in the absence 

 of all positive evidence to the contrary, and possessing a few facts in 

 the affirmative, we will conclude for the present that all the 

 ocelli of Myriapods and Spiders possess a vitreous layer. But 

 the ocelli of Hexapods are undoubtedly homologous with the simple 

 ocelli of Myriapods and Spiders, therefore, they likewise must possess 

 a vitreous layer. Here, also, we have confirmatory evidence, since 

 Grenacher and Giaccio have figured the ocelli of Musca, and Carri- 

 ère, of Polistes gallica, with a vitreous layer which we may assume is 

 always present. 



But what is the relation between the compound and simple eyes? 

 The evidence and the probability is very strongly in favor of the sup- 

 position that all ocelli and compound eyes are dipoblastic; the vitre- 

 ous layer of one, being homologous with the corneal hypo- 

 dermis of the other. The compound e^e must have originated in 

 1 one of two ways; 1) by the fusion of many ocelli, or (2) by the modi- 

 I fication of a single ocellus. 



Let US first form some idea of what the primitive Arthropod eye 

 was like, that we may see in what direction subsequent modification 

 tended to lead it. It is very probable that the aucestral Arthropods, 

 like many Annelids of today, were provided with numerous invaginate 

 eyes situated upon both sides of the head. By the deepening of the cup 

 and ingrowth of the lips, a closed optic vesicle was formed; the entire 

 organ consisted of three layers' (PI. 32, fig. 141); (1) the thick inner 

 wall of the optic vesicle, the retineum; (2) the outer wall, or 

 vitreous layer, secreting the vitreous body ; (3) the corneal hy- 

 po dermis, producing the corneal lens, when one was present. 



The central eyes oi Scorpio (Lankester, PI. 32, fig. 145] and those 



1 We shall not consider the connective tissue fibres which may intervene be- 

 tween the corneal hypodermis and vitreous layer, as a distinct layer. 



