The life cycles of acoels have been determined mostly by in- 

 ference from collection data. For more complete information, 

 laboratory culture of the animals is required, and this has rarely 

 been done. A recent report by Ax (1977) gave an account based 

 on collection data for 14 species of acoels, as well as other 

 Turbellaria, from the North Sea Island of Sylt. He concluded 

 that in 12 of the 14 acoels there is a polyvoltine life cycle (i.e., 

 with more than one generation per year). Also, he noted that 

 "the polyvoltine life cycle is an important prerequisite for the 

 immense population development of several interstitial acoels. 

 In addition the brood size for acoels commonly consists of many 

 eggs:' Apelt (1969) cultured successfully several species of acoels 

 through at least one generation. He followed the embryological 

 development, and his observations agree with those of earlier 

 workers showing that the acoels have a spiral-duet type of 

 cleavage. He also shows the speed of development to be directly 

 related to temperature. Almost all acoels are oviparous but 

 Apelt mentions a few viviparous species. All have a direct type 

 of development without larval stages. See the review by Henley 

 (1974) for a good summary of earlier work on acoel 

 reproduction. 



DIAGNOSTIC CHARACTERS OF 

 THE ORDER NEMERTODERMATIDA 



The order Nemertodermatida includes two genera, Nemer- 

 toderma and Meara, which were originally included in the 

 Acoela, plus a recently defined genus, Flagellophora, which 

 appears to be related to the first two and which is therefore also 

 included in this order. All these species are small forms whose 

 general appearance and habits made it natural when they were 

 first observed to assume they were acoels. However, the 

 uniflagellate sperm in Nemertoderma and Meara (Tyler and 

 Rieger 1975; Hendelberg 1977) and the presence of a gut cavity 

 at least some time during their life history differentiate these 

 genera from acoels. The presence of two statoliths instead of 

 one in the statocyst makes the distinction between nemertoder- 

 matids and acoels relatively easy, the only reported cases of two 

 statoliths in acoels being in fusion and regenerated specimens 

 (Steinbock 1966). The known species of Nemertoderma and 

 Flagellophora are free-living sublittoral forms; Meara is 

 represented by one species from the gut or body cavity of 

 holothurians. 



The size range for nemertodermatids is from ~0.5 to ~3.0 mm. 

 They are generally without conspicuous external features except 

 that in Nemertoderma bathycola the thick outer epithelium has 

 very large vacuoles so that under low magnification the living 

 animal appears to have a shining outer layer. While the other 

 known nemertodermatids are generally oval in form, Nemer- 

 toderma rubra is a typical interstitial form with an elongated 

 body and a coiling, twisting type of movement. 



The digestive tract is a somewhat variable cavity lined by an 

 epithelium that includes two types of cells, amoeboid digestive 

 cells and club-shaped gland ("kornerkolben") cells, with coarse 

 granules in their cytoplasm. These gland cells are considered to 

 be an important characteristic of the order. The mouth, located 

 on the ventral side, may have slightly thickened lips as in Meara 

 or an inturned short section of the epithelium as in 

 Nemertoderma, but in Flagellophora it appears to be lacking 

 altogether. This last genus is characterized by a flagellar organ 

 (Fig. 181a, b) which reaches from the anterior end to the 

 neighborhood of the gut and which may take the place of a 



mouth in securing food, but there is little known about the 

 habits of this species. 



As already noted, the nervous system in the Nemertoder- 

 matida is epithehal or subepithelial. A mass of nerve tissue at the 

 anterior end just under or closely associated with the epithelium 

 represents a cerebral center. Associated with the epithelium all 

 over the body there is a generally diffuse nerve plexus which 

 thins out toward the posterior end and which also shows 

 longitudinal thickenings that probably function as longitudinal 

 nerves. Aside from the statocyst, sense organs have not been 

 described. 



The reproductive systems in the Nemertodermatida are 

 relatively simple and similar to those in the acoels. Testes and 

 ovaries may be represented by a common germinal center or 

 may be clearly separated as compact or follicular units. Female 

 accessory organs are lacking in Nemertoderma and Meara, but 

 in Flagellophora there is a large seminal bursa which opens dor- 

 sally at the center of the body. The male pore is located in the 

 posterior end of the body, ventrally in Flagellophora and ter- 

 minally in Nemertoderma and Meara. In Nemertoderma there is 

 a long male antrum with a seminal vesicle and a mass of 

 associated glands. In Meara the antrum is shorter and the 

 seminal vesicle is smaller and without conspicuous glands. In 

 Flagellophora there is a short male antrum with only a false 

 seminal vesicle. The eggs, as in Acoela, are entolecithal but, as 

 already noted, the sperm are uniflagellate in contrast to the 

 biflagellate sperm of the acoels. 



ECOLOGY 



Little is known concerning the ecology of the acoels and even 

 less about the nemertodermatids for whom we have only the 

 evidence from collection records. Certainly the acoels are more 

 common and occur in a wider variety of habitats than the 

 nemertodermatids. As noted above, our records for the nemer- 

 todermatids are all from sublittoral habitats or, in the case of 

 Meara, as entocommensals from holothurians. The acoels are 

 common subtidally on the surface of sand or sandy mud or 

 intertidally in rock pools and among detritus or algae in shallow 

 water. Neither group can withstand drying and their soft bodies 

 seem to prevent their burrowing within soft mud deposits; 

 relatively few species have been found associated with the inter- 

 stitial fauna of sandy beaches ahhough more are being described 

 from this habitat as it is being explored further. A few species of 

 acoels are pelagic and some acoels have been classed as parasitic 

 (probably commensals) since they are found on, or in the body 

 spaces of, larger animals. 



Laboratory studies which determine accurately the kind and 

 range of factors to which these animals react await their culture 

 in vitro. The recent work by Apelt (1969), which has been men- 

 tioned above, gives the best example of quantitative data. 

 Observations on behavior are found scattered in the literature. 



The role played by acoels in animal communities may be 

 inferred from their food relationships as well as from the 

 numbers present in any one area. Food items may be determined 

 in some specimens from remains found in the digestive paren- 

 chyma. Many species of acoels seem to be predators or 

 scavengers, much the same as other turbellarians. In spite of 

 small size and fragile bodies acoels may be fast swimmers; I have 

 observed Neochildia fusca moving very quickly to overwhelm 

 prey such as small worms, crustaceans, and other small in- 

 vertebrates. When young, members of this species are found to 



