Marine Flora and Fauna of the Northeastern United States. 

 Turbellaria: Acoela and Nemertodermatida 



LOUISE F. BUSH' 



ABSTRACT 



This manual contains an inlroduction lo the general biology, an illustrated key to the genera, and an anno- 

 tated systematic list of the orders Acoela and Nemertodermatida. The key facilitates identification of 16 families 

 and 75 genera of Acoela and includes the 3 genera of the order Nemertodermatida. I he systematic list includes 

 173 species which have been described from the North Atlantic, including some more southern species that might 

 be encountered here, and gives the habitat and known distribution for each species. 



INTRODUCTION 



Acoel turbellarians are almost exclusively marine, and only 

 two species, Oligochoerus linwophilous Ax and Dorjes, 1966 

 and Limnoposthia polonica Kolasa and Faubel, 1974, have 

 been described from freshwater and none from any terrestrial 

 habitat. They are common on various substrata, both inter- 

 tidally and subtidally, on the continental shelf. A few are 

 pelagic, but none have been reported to date in deep seas. 

 Most of the work on this order, aside from the series of papers 

 by Marcus (1950, 1952, 1954) in Brazil, has been carried out by 

 scientists working on collections made in Europe, and new 

 species and genera from these areas are being added even to- 

 day. Relatively few species have been described from the 

 northeast coast of the United States, but experience and 

 materials collected thus far indicate that many, if not most, of 

 the genera present in the eastern North Atlantic are also 

 represented in the western North Atlantic. Therefore, in order 

 for this publication to be most useful in the northeastern 

 United States, I have included all known genera from the 

 world in the key and all genera and species from the North 

 Atlantic are included in the systematic list. The known species 

 of the order Nemertodermatida, all of which are marine, are 

 also from the North Atlantic. 



The position of Acoela in the class Turbellaria has undergone 

 various changes and, while almost always being recognized as a 

 natural division, they have been ranked variously as a subclass 

 (Graff 1904a, b), a suborder (Karling 1940, under order 

 Archoophora), or an order (Ax 1956; Dorjes 1968a; Karling 

 1974). The ranking of Acoela as a separate order has the advan- 

 tage of conforming to the common usage of the term "acoel" 

 and is so used here. The separation of the order Nemertoder- 

 matida from Acoela is a more recent development as is 

 explained below. 



The subdivisions of Acoela have not been generally agreed 

 upon. Graff (1882) recognized two families, Proporidae and 

 Aphanostomidae (= Convolutidae). Luther (1912) retained 

 Proporidae and Convolutidae as major subdivisions of the 

 group based on the presence or absence of a seminal bursa, and 

 this arrangement into two principal families was more or less 

 followed by other workers until the series of papers by Westblad 



'George M. Gray Museum, Marine Biological Laboratory, Woods Hole, MA 

 02543. 



appeared (1940, 1942, 1945, 1946, 1948). Westblad in his final 

 paper proposed the division of Acoela into three tribes; 

 Opisthandropora-Abursalia, Proandropora-Abursalia, and 

 Proandropora-Bursalia, with a series of families included in 

 each tribe. As the names imply, these were based on the position 

 of the male genital pore and on the presence or absence of the 

 seminal bursa. Recently, Dorjes (1968a), in his comprehensive 

 survey of Acoela, criticized Westblad's arrangement in detail 

 and simply divided the order into 1 5 families, based primarily on 

 the structure of the male organs. This avoids at least some of the 

 problems and inconsistencies of Westblad's system and is the 

 arrangement, with the following exceptions, which I have used 

 in this paper. An additional family, Antroposthiidae, has been 

 defined recently by Faubel (1976) and is included here as the 

 16th family. Also, it must be noted that two of Dorjes' families, 

 Hofsteniidae and Nemertodermatidae, have been considered by 

 some authors to deserve ranking as orders separate from 

 Acoela; recent discussions by Ax (1961), Karling (1967, 1974), 

 Tyler and Rieger (1977), and Faubel and Dorjes (1978) lead me 

 to agree that certainly Nemertodermatidae do not belong in 

 Acoela since they have a gut cavity during at least part of their 

 life cycle and also have unitlagellate sperm which are distinctly 

 different from those of Acoela. Specimens of Neinertoderma 

 sp. taken by me in Vineyard Sound near Woods Hole, Mass., 

 clearly show a gut space in some of the specimens. Observations 

 on Meara (Westblad 1949), and on Flagetloplwra (Faubel and 

 Dorjes 1978), which have open guts and characters close to 

 Neniertoderma, also indicate that the family should be in a 

 separate order. 1 concur with the proposals for an order Nemer- 

 todermatida (Westblad 1947; Karling 1940), and am placing 

 Nemenoderma. Meuru, and Flagellophora in the order Nemer- 

 todermatida. 1 am indicating the possibility of Hofsteniidae 

 deserving placement in a separate order by a footnote in the key. 

 Although future work may be expected to add to the families, 

 genera, and species of Acoela, publication of this key to families 

 and known genera should be useful to workers in our area and 

 encourage students on this side of the Atlantic to further work 

 on the Turbellaria in general and Acoela in particular. 



DIAGNOSTIC CHARACTERS OF 

 THE ORDER ACOELA 



The order Acoela, as the name implies, comprises those 

 members of the class Turbellaria which do not have clearly 



