KEY TO THE FAMILIES AND GENERA OF ACOELA AND NEMERTODERMATIDA 



OF THE WORLD 



This key includes the known families and genera of the world 

 since the acoels in the western North Atlantic are poorly known, 

 and anyone working with material from the northeastern United 

 States will need to start by comparing specimens with the 

 families and genera already described from other areas. Par- 

 ticularly pertinent are the genera from the eastern North Atlan- 

 tic since we already know that many of the same genera and 

 some of the same species also occur here. New genera are being 

 defined from time to time, and some appearing very recently, 

 since the key was completed, are added by means of footnotes at 

 appropriate places. 



The arrangement of the famihes and parts of the key are 

 based on DOrjes (1968a) and the debt owing to him is gratefully 

 acknowledged, but I am responsible, of course, for the key and 

 systematic list as here presented. 



In identifying material it is most desirable to start with a study 



of living specimens slightly flattened under a coverslip; the iden- 

 tification to genus often can be made in this way. When this 

 does not suffice, the only recourse is to fix the animals and make 

 sections as suggested on page 7. Unfortunately, in my experi- 

 ence, whole mounts satisfactory for identification purposes are 

 very difficult to attain. The inclusion of a complete diagram for 

 at least one species in each genus will facilitate identification and 

 the annotated systematic list of species for each genus will pro- 

 vide further clues. Page references in the key after each family 

 refer to its location in the systematic list. All figures in the key 

 have been redrawn from the original descriptions so far as possi- 

 ble and the type species for each genus is included in all but a 

 few cases. Except where otherwise noted, all figures are sagittal 

 reconstructions of the entire animal or of male and female 

 accessory organs. Arrows on the figures indicate characters 

 listed in the key. 



Statocyst absent — three instances are reported where statocyst is lacking in species otherwise having the char- 

 acters of Acoela: Amphiscotops sargassi Hyman from Bermuda; Amphiscolops evelinae Marcus from Brazil; 

 Haplodiscus piger Weldon from the North Atlantic (Fig. 4). 



Figure 4. — Haplodiscus piger, dorsal view. 



1 Statocyst present 2 



2 (7) Statocyst with more than one statolith (Fig. 5) 



Order NEMERTODERMATIDA . 



Figure 5. — Anterior end, dorsal view. 



.87 



Figure 6. — Anterior end, dorsal view. 



2 (J) Statocyst with only one statohth (Fig. 6) . 



.Order ACOELA 3 



