Life Cycle: Infection of the rainbow clam first intermediate host (Eurynia iris) has not been observed. It is likely, however, that the miracidia are swept through the incurrent siphon into the branchial chamber, where they attach to the gonad and burrow into it. Branching sporocysts develop in gonads, often completely filling them. While only a single generation has been reported, it seems probably that a mother sporocyst and daughter sporocycst occur. Early development of the cercariae in the sporocysts is rapid but the time required for reaching maturity is unknown. They emerge from the gonads into the branchial chamber. Currents of water flowing from the excutten siphon carry them away from the clams. Upon reaching the outside, they spread the long furcae and hang with the body downward swimming slowly. When the fish swim by, the furcae become entangled with the fins, holding the cercariae in a position that enables them to attach by the anterior holdfast and burrow into the tissues. As they work their way rapidly into the fins, the tail is discarded. Infection of rock bass takes place when fish harboring the cysts are eaten. Upon being released by the action of the digestive juices, the metacercariae migrate into the cecal pouches and mature in 30 days. Adult worms containing up to 200 eggs in various stages of development occur in the cecal pouches of a high percentage of the rock bass. Eggs near the genital opening contain fully formed miracidiae ready to hatch upon reaching the water. Hatching takes place quickly when they reach the water. The miracidium forces the operculum from the egg, emerges, and swims rapidly away (Olsen, 1974).
The form of the body is cylindrical, rounded posteriorly, truncate anteriorly. The terminal anterior surface is flat or slightly convex. The head appendages seven in number, are located on the margin of the anterior surface. These head appendages appear first as short stalks with knob-like ends. Later a small distal and a larger proximal protuberance appear on the underside of the fimbriae near its base. Complete extension then produces long, semi-rigid, downward-curving fimbriae with two finger-like processes near the base of each, the proximal process appearing twice as long as the distal. The large spheroidal anterior sucker extends from the ventral surface nearly to the dorsal surface, occupying most of the space at the anterior end of the body. The body is covered with a thin cuticle in which are embedded flat, blunt-pointed posteriad directed spines inserted acutely on a transversely elongate basal plate. The spines are more prominent in the anterior region and cover the entire lining of the anterior sucker, indicating that the sucker is formed as a depression on the ventral surface. Elongated subcuticular unicellular glands are uniformly distributed over the surface of the body be-neath the cuticle and body-wall muscles. The gut of B. elegans is more nearly oval than in B. papillosus previously described (Woodhead, 1929) and is relatively larger, measuring 0.112 to 0.160 mm in length. The rest of the anatomy of the adult B. elegans is so similar to B. papillosus that further description is unnecessary. B. elegans has the largest egg of the three species of gasterostome thus far investigated. Eggs of older individuals were fewer in number and larger than eggs of younger specimens. A mature worm secured from an experimentally infected blue-gill, Lepomis pallidus, contained only 150 eggs. All were of a very uniform size and none were malformed. Discharged eggs of this individual measured 0.046 to 0.048 mm by about 0.021 mm. The number of eggs in other individuals ranged from 50 to 250, the usual number being in the neighborhood of 200. Measurements of over 100 discharged eggs from different individuals showed a length of 0.040-0.050 mm, average 0.047 mm, and a width of 0.018 to 0.022 mm, average 0.021 mm.
Sources: Olsen O.W. 1974. Animal parasites, their life cycles and ecology. University Park Press, London, Baltimore & Tokyo, 3rd Revd Edn., p. 261.