contain diatoms almost exclusively, but they shift to the role of 

 active predators as they grow larger and mature. In some 

 habitats acoels are only a small part of the biological community 

 structure, but often they are present in enormous numbers (Ax 

 1977). In such cases they must play a significant role in the food 

 web of the community. 



COLLECTING AND EXAMINATION METHODS 



The small size and rather fragile body structure of both the 

 acoels and the nemertodermatids present difficulties in their col- 

 lection and study. Both their fragility and small size result in 

 most species not being recovered by general sampling techniques 

 such as benthic nets or sledges. Relatively undisturbed samples 

 of bottom materials, algae, or other plant materials which are 

 brought into the laboratory and processed there give the best 

 results. The most effective methods for processing are: 1) allow- 

 ing natural concentration of the specimens at the surface of 

 bottom samples or at the surface of the overlying water after 

 standing, 2) washing out or sieving of specimens with or without 

 the use of narcotic solutions, and 3) seawater ice treatment. A 

 few of the larger or more conspicuous species (e.g., Polychoerus 

 caudatus, a bright orange species) may be collected by hand 

 from stones or shells in the intertidal zone. The seawater ice 

 method has been shown to be superior for quantitative analysis 

 of interstitial species (Uhlig et al. 1973). 



Some species which normally live on or near the surface of the 

 substratum tend to go downwards when disturbed and many 

 become mixed with the substratum in spite of care in collecting. 

 If the substratum samples or masses of algae or other plants are 

 allowed to stand in the collecting water in the laboratory, 

 however, many of the animals concentrate at the surface of the 

 substratum or at the surface of the water. The length of time 

 necessary for this to occur may vary from a few hours to a day 

 or so. Apparently, the animals are reacting to lack of oxygen in 

 lower layers, although change of temperature may also be 

 involved. They may be picked up from the surface with a 

 medicine dropper, either with or without the aid of a dissecting 

 scope. 



Washing out of specimens from the substratum may be done 

 in several ways. Vigorous shaking of algae or other plant 

 material in seawater will dislodge some specimens. Vigorous stir- 

 ring of sand samples in seawater followed by quick decanting of 

 the water as sand particles settle will recover many specimens 

 since neither the acoels nor the nemertodermatids have the 

 strongly developed adhesive organs found in some other 

 turbellarians. For most interstitial organisms a narcotizing solu- 

 tion (usually 7% MgCl2 in tap water) is used so that the animals 

 may be passively washed out by stirring and decanting, by siev- 

 ing, or by elutriation of the material in a separation funnel (see 

 Hulings and Gray 1971, for an account of various methods). On 

 replacement of the narcotizing solution the animals recover 

 readily if they have not been left in the narcotic too long, but 

 acoels tend to die very soon and also to break up when handled 

 in the relaxed state. If the material is fixed and stained with a 

 general stain, such as Rose Bengal, before washing out, this dif- 

 ficulty is avoided, but the process of sorting afterwards is much 

 more difficult since most turbellarians, and especially acoels and 

 nematodermatids, look like small oval lumps after fixation and 

 can be easily mistaken for broken bits of tissue or for ciliates 

 which may be about the same size. 



The seawater ice method is an effective method (Uhlig et al. 

 1973) for recovering Turbellaria brought into the laboratory 

 with sand or sandy mud and is being adopted by many workers 

 who desire a quantitative analysis of such material. This method 

 involves the use of a temperature and water salinity gradient to 

 induce the animals to leave the substratum. A simple apparatus 

 (Fig. 3) is set up consisting of a tube to hold the substratum 

 samples plus seawater ice which provides a slight temperature 

 gradient but, more importantly, results in a change from low to 

 high salinity of the water that runs through the sample as 

 melting occurs. The live animals collect in the dish at the bottom 

 of the apparatus and can then be sorted and prepared for fur- 

 ther study. The chief drawback of this method is that it is 

 somewhat time consuming and may not be worthwhile if only a 

 quick determination of the most prominent members of a fauna 

 is desired. 



tube holcJer- 



insulation. 



secJiment. 

 culture (dish 

 with sea water. 



V / l\/ 



X 



.plastic tube 



sea-water-ice 



nylon gauze 



rubber bancj or 

 ig 

 dish 



/^ ruDoer oan 

 ri---^^'^ plastic rin( 



I n petri 



.overflow 



Figure 3.— Apparatus for seawater ice treatment of sediment (redrawn from Uhlig et al. 1973). Use of the insulation layer is optional. 



6 



