The status of bluegill (Lepomis macrochirus) and green sunfish (Lepomis cyanellus) as homologous hosts for the acanthocephalan Neoechinorhynchus cylindratus was experimentally determined. It was found that the adult parasite did not establish in bluegills, but that these fish could serve as paratenic host. In contrast, complete growth and development to the adult stage occurred in the green sunfish. When green sunfish were intubated with 10 cystacanths/fish, the parasite exhibited a clear preference for the anterior half of the intestine; when 50 cystacanths/fish were intubated, the parasites showed a preference for the posterior half of the intestine. With repeated exposure of cystacanths, the parasites were distributed throughout the intestine. The extent of histopathology induced by N. cylindratus was related to the numbers of parasites present. In light infections (10 cystacanths), the parasite penetrated deeply into the intestinal wall and connective tissue developed around the proboscis. In infections with 50 cystacanths, the proboscis penetration was shallow and little if any connective tissue accumulated. There was also an indication that in crowded areas, the parasites appeared to change their sites of attachment frequently. In both heavy and repeated infections, the parasites evoked significant goblet cell hyperplasia and substantial quantities of mucus covered the intestinal wall. It is suggested that the sticky covering and the presumed presence of antibodies in the mucus combined to create a protective barrier thereby reducing the numbers of parasites that could attach and become established.

Distribution of Leptorhynchoides thecatus (Acanthocephala: Rhadinorhynchidae) among the pyloric caeca and the relationship between site and rate of maturation were studied in laboratory infectons of 10, 25 and 40 cystacanths fed to green sunfish, Lepomis cyanellus. After 1 week fish fed at each intensity had significantly different numbers of worms. By the 3rd week post-infection, parasites disappeared from the anterior portion of the intestine. At this time the mean numbers of worms recovered from 25 and 40-cystacanth infections were not significantly different. At the end of the 1st week, the area where caeca join the alimentary tract (between caecal area) and caeca numbered 6 and 7 contained significantly more worms than the other sites. By the 3rd week post-infection only caecum 7 contained significantly more worms, and at 5 weeks there was no significant difference between the number of worms present in any caecum or the between caecal area. Initially worms in the more intense infections matured more slowly, but by the 3rd week post-infection there was no significant difference in the states of maturity. The rate of maturation was not related to the site occupied.