A CONTRIBUTION TO THE EMBRYOLOGY AND 

 PHYLOGENY OF THE PYCNOGONIDS. By 



T. H. MORGAN, ^ellow of Johns Hopkins University. With 

 Plates I, II, III, IV, V, VI, VII and VIII. 



TABLE OF CONTENTS. 



Part I. 



Introduction 1 



Embryology 3 



Tanystylum and Phoxichilidium 4 



Pallene empusa 8 



External changes 8 



Internal changes 15 



Comparisons 22 



Phylogeny 25 



Criticism of Dohrn and Hoek 25 



Conclusions from Embryological Data 28 



The " Pantopod-larva " 33 



Conclusions 34 



Part II. 

 The Metamorphosis of Tanystylum 36 



Part III. 



The Structure and Development of the Eyes of Pycnogonids 49 



The Anatomy of the Adult Eye 49 



Histology of the Eye 56 



The Development of the Eyes of Tanystylum 59 



Comparison with the Arachnid Simple Eyes 63 



Inversion of the Eye 65 



General Consideration of the Simple Eye 69 



Reference to Literature 71 



Description of Plates 72 



PART I. 



In the year 1767 Karl Linne, in the twelfth edition of his 

 Systema Naturae, described under the name of Phalangium a 

 Pycnogonid, and here for the first time is the question raised 

 whether the group is to be ranged under the Arachnids or the 

 Crustacea. A hundred years elapsed and the problem remained 



