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Two cystidicolid nematodes, Ctenascarophis lesteri n. sp. and Prospinitectus exiguus n. sp., are described from the skipjack tuna, Katsuwonus pelamis, from Fiji, New Caledonia, Marquesas Islands, New Zealand, Papua New Guinea, and Puerto Rico. The former species differs from Ctenascarophis gastricus, the only other member of the genus, by having comb rows extending farther posterior than two-thirds the body length, to the anus in the female; it also has differently distributed spines on each comb, with a maximum of 32 rather than 8. The male has 7 rows of precloacal longitudinal crests; 3 pairs of precloacal, 1 pair adanal, and 6 pairs postcloacal papillae; and a spicule ratio of 1:3.4-1:5.0. The latter species differs from Prospinitectus mollis, the only other species in the genus, by being less than one-half as long, 3.5-5.9 mm long; with a proportionally longer esophagus; fewer spines per spine ring, a maximum of 49 compared with 100; deirids that lack spinules; males with an unornamented precloacal cuticle as well as a different number and distribution of caudal papillae, 2 precloacal and 7 postcloacal; and a smaller spicule ratio, 1:4 rather than 1:5-1:7.

Heliconema brooksi n. sp. is described from the shrimp eel, Ophichthus gomesi, in Mississippi Sound. This species is distinguished from other members of the genus by possessing in the male 40 or more tessellated longitudinal ridges, a spicule ratio averaging 1:10.6, 4 precloacal papillae, and 6 postcloacal papillae. Females have a protruding vulvular flap. Both sexes have a prominent sclerotized cardium. Heliconema brooksi is morphologically most closely related to Heliconema heliconema. A juvenile worm from the white shrimp, Penaeus setiferus, in the same locality may be the infective stage.

Two cystidicolid nematodes, Ctenascarophis lesteri n. sp. and Prospinitectus exiguus n. sp., are described from the skipjack tuna, Katsuwonus pelamis, from Fiji, New Caledonia, Marquesas Islands, New Zealand, Papua New Guinea, and Puerto Rico. The former species differs from Ctenascarophis gastricus, the only other member of the genus, by having comb rows extending farther posterior than two-thirds the body length, to the anus in the female; it also has differently distributed spines on each comb, with a maximum of 32 rather than 8. The male has 7 rows of precloacal longitudinal crests; 3 pairs of precloacal, 1 pair adanal, and 6 pairs postcloacal papillae; and a spicule ratio of 1:3.4-1:5.0. The latter species differs from Prospinitectus mollis, the only other species in the genus, by being less than one-half as long, 3.5-5.9 mm long; with a proportionally longer esophagus; fewer spines per spine ring, a maximum of 49 compared with 100; deirids that lack spinules; males with an unornamented precloacal cuticle as well as a different number and distribution of caudal papillae, 2 precloacal and 7 postcloacal; and a smaller spicule ratio, 1:4 rather than 1:5-1:7.

Heliconema brooksi n. sp. is described from the shrimp eel, Ophichthus gomesi, in Mississippi Sound. This species is distinguished from other members of the genus by possessing in the male 40 or more tessellated longitudinal ridges, a spicule ratio averaging 1:10.6, 4 precloacal papillae, and 6 postcloacal papillae. Females have a protruding vulvular flap. Both sexes have a prominent sclerotized cardium. Heliconema brooksi is morphologically most closely related to Heliconema heliconema. A juvenile worm from the white shrimp, Penaeus setiferus, in the same locality may be the infective stage.

The distribution and abundance of the nematode Camallanus oxycephalus infecting white bass, Morone chrysops, in western Lake Erie was studied for over 2 years. Infection was generally more frequent and of higher intensity in large fish. The frequency distributions of nematode abundance in all segments of the fish population followed the negative binomial distribution. The data show seasonal cycles in population structure, site selection, intensity of infection, maturation, and reproduction. Infection occurs during July and August with a resulting peak in population density; during late summer and autumn, mortality, probably density-dependent, reduces the population by 30 to 60%; surviving worms are eliminated at 1 year of age. Growth and development of female worms is arrested from November to April, then proceeds at a rapid rate until the worms release their larvae and die. This growth pattern is probably related to temperature but may also involve host hormone cycles. The dispersal period of the nematode coincides with the annual maximum density of the intermediate host, a cyclopoid copepod,and is interpreted as an adaptation which increases the probability of successful transmission. Because the number of larvae produced by each female worm is a function of body volume, natural selection has favored rapid spring growth and attainment of large body size relative to the male worm. Both seasonal timing in the life cycle and the number of larvae produced are important factors in determining the abundance of this and perhaps other parasites. Evidence is presented suggesting that fluctuations of environmental parameters may disrupt the timing of transmission and alter the distribution and abundance of the parasite. It is hypothesized that the magnitude of such changes in parasite abundance may be related to the complexity of the host-parasite system.

A total of 178 channel catfish (Ictalurus punctatus), 162 freshwater drum (Aplodinotus grunniens), 60 smallmouth bass (Micropterus dolomieui) and 2,488 yellow perch (Perca flavescens) were examined for larval Eustrongylides tubifex (Nitzsch 1819) Jägerskiold 1909 during 1973 and 1974. Older, large fish exhibited higher prevalence, larger mean numbers of larvae in infected fish, larger mean numbers of larvae in all fish examined, and wider range of intensity of infection. Infections were most extensive in yellow perch. The majority of larvae were removed from mesenteric cysts. Under experimental conditions, larvae migrated from cysts through the body wall when environmental temperatures reached or exceeded 40 C. Fourth-stage larvae removed from yellow perch successfully reinfected a variety of poikilothermous experimental hosts. Both young-of-the-year and I+ age/size class yellow perch exhibited increasing monthly larval worm burdens during portions of the sampling period. Older (II+) age/size class yellow perch did not exhibit significantly increasing worm burdens. No significant differences were noted in numbers of male and female larvae or in worm burdens in male and female yellow perch.

Eustrongylides tubifex (Nitzsch 1819) Jägerskiöld 1909 is redescribed from six male and 10 female adult specimens from experimental infections in mallards (Anas platyrhynchos). Our redescription is compared with that of Jägerskiöld (1909) and those of the Russian investigators Kontrimavichus and Bakhmeteva (1960) and Karmanova (1968). Our redescription differs in the ranges of total body length, esophageal length, maximum width, spicule length and vagina length. The cuticular fringe and inner rim of small papillae of the male bursa are shown to be definitive species characters. A key to the species of Eustrongylides is included.

Three hundred and thirty specimens of 7 species of passerine birds from South Bass Island, Ottawa County, Ohio, were examined for helminth parasites. The total number of helminth specimens collected was 4,333. Forty-one helminth taxa were identified. Ten species of helminths were identified as having foci of infection on the island. An index of association for these 10 species is presented. The low association revealed between helminth species utilizing common species of intermediate hosts indicated that a single intermediate host specimen is likely to harbor infective stages of only one species of parasitic helminth. An index of similarity is presented to express the species importance relationships of the helminth faunas of the 7 species of birds: red-winged blackbirds (Agelaius phoeniceus), blue jays (Cyanocitta cristata), brown-headed cowbirds (Molothrus ater), common grackles (Quiscalus quiscula), house sparrows (Passer domesticus), starlings (Sturnus vulgaris), and robins (Turdus migratorius). The results reveal that competition between these avian species for invertebrate food resources helps to maximize the transmission of those helminth populations which utilize these same invertebrates as intermediate hosts. The aggregation of birds into mixed feeding flocks insures a heavy loading of the feeding grounds with agents infective for the invertebrate populations.

The distribution and abundance of the nematode Camallanus oxycephalus infecting white bass, Morone chrysops, in western Lake Erie was studied for over 2 years. Infection was generally more frequent and of higher intensity in large fish. The frequency distributions of nematode abundance in all segments of the fish population followed the negative binomial distribution. The data show seasonal cycles in population structure, site selection, intensity of infection, maturation, and reproduction. Infection occurs during July and August with a resulting peak in population density; during late summer and autumn, mortality, probably density-dependent, reduces the population by 30 to 60%; surviving worms are eliminated at 1 year of age. Growth and development of female worms is arrested from November to April, then proceeds at a rapid rate until the worms release their larvae and die. This growth pattern is probably related to temperature but may also involve host hormone cycles. The dispersal period of the nematode coincides with the annual maximum density of the intermediate host, a cyclopoid copepod,and is interpreted as an adaptation which increases the probability of successful transmission. Because the number of larvae produced by each female worm is a function of body volume, natural selection has favored rapid spring growth and attainment of large body size relative to the male worm. Both seasonal timing in the life cycle and the number of larvae produced are important factors in determining the abundance of this and perhaps other parasites. Evidence is presented suggesting that fluctuations of environmental parameters may disrupt the timing of transmission and alter the distribution and abundance of the parasite. It is hypothesized that the magnitude of such changes in parasite abundance may be related to the complexity of the host-parasite system.

Eustrongylides tubifex (Nitzsch 1819) Jagerskiold 1909 is redescribed from six male and 10 female adult specimens from experimental infections in mallards (Anas platyrhynchos). Our redescription is compared with that of Jagerskiold (1909) and those of the Russian investigators Kontrimavichus and Bakhmeteva (1960) and Karmanova (1968). Our redescription differs in the ranges of total body length, esophageal length, maximum width, spicule length and vagina length. The cuticular fringe and inner rim of small papillae of the male bursa are shown to be definitive species characters. A key to the species of Eustrongylides is included.

Morphology is cited as the primary criterion for the identification and classification of parasites. Recent criticisms of the so-called "classical methods" used in parasitological systematics are discussed and the advantages of a broader system maintaining morphology as its basis are presented. Comparative data derived from interdisciplinary studies would be used to strengthen the morphological system. The reasons why parasitologists have not readily adopted the approach of the "new systematics" are discussed, and it is suggested that more studies in speciation and evolution of parasites be undertaken. These studies would furnish comparative data, but they would not change the purpose of systematics in parasitology. In the past the parasitological taxonomist has named, identified, and classified most groups of parasitic organisms using morphology as his primary criterion. The approximate identification of a specimen or a series of specimens was determined by using the literature. Comparisons were then made with previously identified material. If the specimens represented a known taxon, they were equated with the proper categorical level in the existing hierarchy of classification. If the specimens represented a previously unknown group of parasites, they were described and named in an acceptable publication. One of the specimens was designated as the TYPE in order to avoid confusion about the application of the

Rhigonema thysanophora sp. n. (Nematoda: Rhigonematidae) is described from Euryurus erythropygus Brandt, 1841. This is the first species in this genus to be recorded from a polydesmid millipede in North America. The other recognized species in the genus are listed and the characters which separate the new species from others in the genus are elucidated. The specimens used for this report were removed from the posterior part of the intes- tine, the intestinal-rectal valve, and from the anterior part of the rectum of the polydes