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The particle retention spectra of the mucous nets of three species of salps were determined by measuring differences in grazing rates on particles of different sizes with a Coulter Counter. It is pointed out that the only correct method for determining these spectra for an organism feeding in a closed vessel is to compare rates of clearance for each particle size class; if ratios of the quantity ingested to the initial amount' in each size class are used, incorrect retention spectra will be produced. Cyclosalpajoridana, Cyclosalpa afjkis, and Cyclosalpa polae can all remove particles about 4 SL. in diameter and larger with 100% efficiency. Observed differences between species in the retention characteristics of the filtration apparatus can be ascribed to differences in the sizes of the. animals. In general, smaller salps can retain a greater fraction of small particles than can larger ones. Although quantitative differences in the particle retention spectra between generations and species exist, there is little evidence that qualitative differences arc also present. Depending on the abundance of these salps, their grazing impact on particles with diameters as small as 1 p could be significant.

When the pelagic tunicate Pegea confoederata is presented with concentrations of particulate material equivalent to levels found in neritic areas, its feeding mechanism is disrupted. A bolus is formed that blocks the esophagus, dramatically decreasing the amount of material that enters the stomach. This bolus never occurs at particle concentrations ~0.5 ppm, and always forms at particle concentrations >S.O ppm. For oceanic waters, 0.5 ppm lies in the upper range of values, so that it appears that P. confoederata is adapted for feeding on average concentrations of particulate material in the open sea. These results are in contrast to those with most of the Calanoid copepods that have been studied. Because P. confoederuta feeds on oceanic average amounts of food, it can be more evenly distributed throughout the great oceanic central water masses, decreasing the likelihood of its discovery by predators.-

In several recent papers dealing with the feeding of copepods on natural particle assemblages investigators have claimed to show types of behavior more complex than simple sieving by the animals. We question these interpretations because first, the sensing-zone counters used in these experiments size particles on the basis of their volumes, so that the equivalent spherical diameters thus obtained bear little relationship to the linear dimensions of many algal cells; and second, sieves remove particles from water on the basis of their linear dimensions, not necessarily on the basis of their volumes. If mixtures of phytoplankton of diverse shapes are passed through a sieve and sized with a sensing-zone counter, the sieve will appear to have "behavior."Two other sources of artifact-the breakage of phytoplankton cells by experimental agitation and by the feeding of the copepods-add to the difficulties of interpreting natural particle experiments.Proper control experiments to evaluate the contribution of these sources of artifact to experimental results have not yet been done.

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