ON THE OEIGIN OF VEETEBEATES, DEDUCED EEOM 

 THE STUDY OF AMMOCCETES. By Walter H. 

 Gaskell, M.D., LL.D., F.E.S., University Lecturer on 

 Physiology ; Fellow of Trinity Hall, Camhridye. 



Part IX. — On The Origin of the Optic Apparatus; 

 The Meaning of the Optic Nerves. 



In Part V. of this series of papers the explanation given of the 

 origin of the eye muscles assumed that the pineal eyes of the 

 vertebrate were homologous with the pair of median eyes in 

 the Eurypterid, and that the lateral eyes of the vertebrate had 

 arisen directly from the lateral eyes of the Eurypterid ancestor. 

 The time has now come to consider the difficulties connected 

 with this assumption, and to discuss the probable mode of origin 

 of the vertebrate eyes. 



The Nature of the Arthropod Eyes. 



Before commencing this task it is advisable to consider the 

 nature of the eyes of arthropods according to the opinion of the 

 present day. These eyes have been divided into two great 

 groups — the so-called simple and compound eyes respectively. 

 The eyes of arachnids, both median and lateral, the median eyes 

 of insects usually called ocelli or stemmata, and the median eyes 

 of crustaceans all belong to the former group. In fact, the 

 median eyes are in all cases simple, while the lateral eyes may 

 be simple or compound. 



This distinction is not in reality a good one, as it is still an 

 unsettled question whether the arachnid eyes, for example, are 

 not in reality compound in the sense of being composed of a 

 number of ommatidia, though simple in the sense of possess- 

 ing only a single corneal lens. Then again, eyes have been 

 divided into the two groups of facetted and non-facetted eyes, a 

 division which corresponds largely to the compovmd and simple 

 eyes respectively, and yet is not entirely satisfactory because 



