ANATOMY OF ACM.EA TESTUDIXALls MI LIJ-R 



PART I. INTRODUCTORY MATERIAL — EXTERNAL 

 ANATOMY 



M A. WILLCOX 



Introduction 



Some months ago I published in this journal under the title 

 "Biology of AcmoBa testudinalis Miiller" an excerpt from a mono- 

 graph of this species upon which I have long been engaged. 

 I have now decided to publish the entire monograph in sections 

 of which the present is the first. It should naturally be introduced 

 by remarks upon the scope of the work but as these were prefixed 

 to the earlier article, I refer the reader to that, adding merely that 

 the investigation is intended not only as a contribution to a knowl- 

 edge of the New England fauna but also as a first step in such a 

 careful comparative study of the different species of the genus as is 

 there suggested. This being its aim, it occupies itself in the main 

 with anatomy, touching only incidentally upon either histolog}^ or 

 embryology. I have, moreover, tried to make a paper which would 

 serve as an introduction to the study of the neglected but fascinat- 

 ing group of Gastropoda. This aim will, I trust, serve as excuse 

 for the admission of some material too elementary to be included 

 in a paper addressed exclusively to specialists. 



Zoological Position of Acm^a and its Allies 



Acmsea belongs to the Scutibrauchiata (Aspidobranchia, Dioto- 

 cardia of Bouvier et al.), a siihonlcr which includes all the more 

 primitive Prosobranchiata and which i-^ (h\ iMhh' into two sections: 

 the Rhipidoglossa (Diotocanha of Kciny Pci rieri and the Doco- 

 glossa (Heterocardia). The only characteristics by which mem- 

 bers of these two sections may infallibly be distinguished are 

 first, the nature of the radula and second, the presence of dialy- 



171 



