30 THE ANCESTRY OF VERTEBRATES 



they can only serve to distinguish light and dark, and their 

 resemblance to the highly perfected eyes of Vertebrates is 

 very slight. Further differentiation, however, of these primi- 

 tive optic organs leads quite gradually to the varied forms 

 of eyes met with among Annelids and the closely allied 

 Molluscs, some of which very nearly approach the com- 

 plexity of the Vertebrate eye. The structure of the Annelid 

 eyes has been investigated especially by Carriere (1885), 

 Andrews (1891, 1892) and Hesse (1899). The first step 

 towards further perfection is. a depression of the ectoderm, 

 giving rise to so-called pit-eyes, as found especially among 

 sedentary Polychaets. These pits may get very deep and 

 finally close and separate from the ectodermal epithe- 

 lium, thus giving rise to vesicular eyes, as found especially 

 among predatory Polychaets, and which reach their highest 

 perfection in the pelagic Alciopids. Here the eyes, first 

 described by Greeff (1876) and afterwards by several other 

 authors, are large swelling vesicles, provided with an internal 

 lens, a cornea, a vitreous body and even a ring of accomo- 

 dative muscles Next to the Cephalopods the Alciopids, as 

 Hesse (1899, p. 475) remarks, afford the only example of 

 accomodative power among Invertebrates (cf. also HESS, 

 1909, 1914 1). 



In Molluscs a similar, parallel series may be composed 

 (cf Hesse 1902, p. 622) from the pit-eyes found in several 

 primitive Gastropods and in Nautilus to the vesicular eyes 

 of the other Gasteropods and those of Cephalopods, where 

 the organisation reaches a height closely approaching that of 

 the Vertebrate eye to which it shows a resemblance so 

 striking that, ever since, it has attracted the attention of 

 zoologists. Yet it cannot be doubted that this complex 

 structure has developed out of the paired pigment spots 

 which we find also in the trochophoras of primitive Molluscs, 

 such as Amphineura, Lamellibranchiata and several Gas- 

 tropoda. The points of resemblance between Cepha- 

 lopod and Craniate eyes are no more striking, however, than 

 is the difference between them: the successive layers of the 

 retina and the optic ganglion applied to it in the one case 

 showing the reverse arrangement from those in the other. 



') A third case has since been described by Hess and Gerwerz- 

 HAGEN, in Pterotrachea (1914). 



