216 
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 
Moilluscs, some of which. very nearly approach the com- 
plexity of the Vertebrate eye. The structure of the Annelid 
eyes has been investigated especially by CARRIÈRE (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, iere with an internal 
lens, a cornea, a vitreous body and even aring 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, 
Wi 1914 *). 
olluscs 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. 
1) A third case has since been described by Hess and GERWERZ- 
HAGEN, in Pterotrachea (1914). 
