10 ' THE MEDUSAE 



and Indian oceans, the eastern half of the Pacific still remains practically a 

 mare incogniium in this respect. 



The collection (20 species) made in Acapulco Harbor is likewise important 

 from the distributional standpoint, for the Medusa fauna of the west coast of 

 Central America was previously almost unknown. When compared with 

 the West Indian Medusae it illustrates in striking fashion the close relationship 

 between the littoral faunae on the two sides of Central America. 



Owing to the rapid growth of our knowledge of the structure and relation- 

 ships of the Medusae the classification of both Acraspedae and Craspedotae is 

 in a state of such constant change that no system as yet proposed can be ex- 

 pected to remain permanent. The scheme adopted in the present memoir is 

 based in its essentials, as all future attempts must be, on the classifications 

 outlined by Gegenbaur, Agassiz, and Haeckel. But I have not hesitated to 

 adopt the very extensive modifications which recent authors have shown to 

 be necessary ; and several new changes are proposed, to which attention will 

 be called in the appropriate connections. 



Perhaps a few words are called for to explain a presentation of the 

 subject-matter in the reverse of the so-called " natural order " now usually 

 adopted. The present arrangement was adopted for purely practical reasons 

 connected with the preparation of the drawings and plates, and for the sake 

 of preserving a parallel arrangement of text and plates; it is not to be taken 

 as indicating the author's idea of phylogeny. Within the several orders, 

 where supposed relationships rest on a much firmer basis than they do between 

 the orders, I have preserved what I believe to be the natural sequence of 

 families and genera. 



Throughout the cruise, the care and preservation of the Medusae fell to 

 my special lot, so that I had the opportunity to study the great majority of the 

 specimens in life, or at least in the fresh condition ; a privilege of the greatest 

 value, as any student of this group wiU recognize. I was also able to prepare 

 colored sketches of many of the species, a few of which are reproduced in 

 this memoir. Colored drawings of Atolla and Ptychogena were likewise made 

 by Mr. A. M. Westergren, artist on the expedition. 



The great majority of specimens were preserved in 5% formalin in sea 

 water, a method which has proved satisfactory both for gross anatomy and 

 for histology, its only drawback being that otoliths are frequently dis- 

 solved. A few individu.ils were preserved in alcohol, and still others in 

 various of the acid histologic fixing reagents. I may call attention here to 



