RELATION OF ARTHROPOD HEAD TO ANNELID PROSTOMIUM. 247 



On the Relation of the Arthropod Head to the 

 Annelid Prostomium. 



By 



Edivin S. Ooodi'icli, B.A., 



Assistant to the Liaacre Professor, Oxford. 



The question of the segmentation of the Arthropod head, 

 and of the homology of the preoral region in Arthropods and 

 Annelids, has for long excited the interest of naturalists, 

 giving rise to much discussion, and leading investigators to 

 the discovery of many important facts. Tiie present paper, 

 written at the suggestion of Professor E. Ray Lankester, does 

 not claim to be a contribution to our knowledge of the pro- 

 blems involved, nor an exhaustive history of the subject; it 

 aims neither at originality nor completeness, but is merely an 

 attempt to give a plain account of the questions at issue, and 

 the advance that has been made towards answering them, for 

 the benefit of those who have not devoted special attention to 

 the subject. 



If we wish to compare the preoral region of an Arthropod 

 with that of an Annelid, it is necessary first of all clearly to 

 understand the relation of the prostomium and the peri- 

 stomium, or buccal segment, to each other, and to the other 

 segments of the body of an Annelid worm. 



It was Professor Huxley who first introduced the word 

 prostomium in his ' Lectures on General Natural History,' 

 published in 1856 (6). ^'The body of the Polynoe,'' says 

 Huxley, ''is composed of a series of twenty-six '^ somites," 

 terminated anteriorly by a 'segment,' the prestomium^ 

 (Kopf-lappen, Grube), and posteriorly by another, the pygi- 

 ' Tne modified form prostomium was introduced by Lankester (8). 



