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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The American Society of Parasitologists is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Parasitology.Much has been learned of the biology of cestodes since Clark Read published his paper in 1951, yet in many respects our knowledge remains distressingly meager. Read observed that, in Hymenolepis diminuta and several other species, the size attained by the worms in their definitive host was roughly inversely proportional to the number of worms present, a phenomenon he called the "crowding effect." The adaptive value of the crowding effect seems self-evident: the size of the individual worms is limited, by whatever means, to an aggregate mass that the host can tolerate without adverse consequences. The known list of cestodes that demonstrate a crowding effect has increased little in the past 40 yr, and a definitive explanation for the effect's mechanism of operation remains elusive. Inasmuch as development and growth of the worms is being controlled by some means external to the individuals in the population, the mechanism of this control is of considerable biological interest.In their accompanying paper, Bush and Lotz (1999) point out that ecological consideration of crowding can relate to predator-prey relationships or to competition. Because we can disregard predator-prey relationships, I focus here on competition, which in turn can take the form of exploitative competition (required resources reduced in supply by the organisms) or interference competition (substances released by organisms having an adverse effect on others in the population). I also examine the possible role of the host immune response in the crowding effect. Exploitative competition?Interference competition? Roberts (1961) suggested that some compound(s) excreted by the worms could adversely affect growth of other worms in the population. When crowded worms were recovered from the * Invited Review to complement: C. P Read, 1951, The "crowding effect" in tapeworm infections. Journal of Parasitology 37: 174-178.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Species reduced to synonymy are E. osburni Tidd and Bangham, 1945 (with E. celestis); E. nigritus Wilson, 1916 (with E. centrarchidarum); E. confusus Bere, 1931 (with E. luciopercarum); and E. skriabini Mueller, 1936 (with E. luciopercarum). A key to the species in the genus based on characteristics of the adult females is presented. A generic diagnosis, specific diagnoses, and some comments relative to each species are included.The genus Ergasilus has not received comprehensive attention in the United States since Wilson (1911), although its importance is attested by the numerous reports in fish parasite surveys since then (see Hoffman 1967). In addition, there have been several new species described since Wilson's monograph (). Some of these new species have been synonymized with previously described species (Mueller 1936b), and some of the synonyms have been redescribed as distinct (Roberts 1 This study was supported in part by NIH grants AI-06153 and 5 TOI-AI-226. I am indebted to the following persons for the loan or gift of specimens: 135 1969a). Many of the earlier descriptions are either incomplete, inaccurate, or both, and partly for this reason there have been numerous errors in the survey reports. Other reasons for these errors were expressed succinctly by Pennak (1963) in reference to other cyclopoids. Therefore, it appeared desirable to restudy all known species of the genus occurring in North America. It is not anticipated that the present treatment will be definitive, but it will be of value as a basis for further work.Methods and terminology are as used previously (Roberts 1969a,b). The key relies chiefly on characteristics which can be observed with a minimum of dissection. The reader is referred also to the dissection and preparation instructions given by Pennak (1963) for the free-swimming cyclopoids, most of which can be applied to Ergasilus. For identification, it is usually necessary to mount at least one specimen whole, dorsal side up, under a supported cover slip, and to perform at least some dissection on one specimen, ventral side up, i.e., to remove the antennal area (including antennae and antennules) and the genital segment plus abdomen (including the 4th free thoracic segment bearing the 5th legs). The egg sacs, if any, should be separated from the genital segment before that segment is removed from the rest of the thorax.In collecting Ergasilus specimens, it is generally preferable to excise and preserve the fish gill arches entire. The specimens mav be removed from the gill filaments later under a dissecting microscope to avoid damaging the antennae.Considerable emphasis is placed in the kev and diaenoses on the prehensile antennae. Functioning to h...