|330 HETEROCHRONY AND PAEDOMORPHOSIS
reaches its terminal host when a fish ingests the infested worm (Wisniewski, 1928; Mackiewicz, 1972). Some species maintain a capacity for facultative reproduction in the oligochaete (Calentine, 1964, on A. iowensis, which may mature in the seminal vesicles of Limnodrilus hoffmeisteri). In other species, maturation in the oligochaete has become obligatory; "appropriate" fish will not be infected even if they eat parasitized worms (Nybelin, 1963, on A. sieboldi; Kennedy, 1965, on A. limnodrili).
The cestodarian genus Amphilina is parasitic in the coelom of fishes and tortoises. Since these are common sites of some eucestodan larvae (and since Amphilina maintains a larval morphology), Cheng (1964, p. 302) regards the genus as progenetic. In a not implausible bit of paleopoetry, Janicki (1930) speculates that the ancestors of A. foliacea, now a parasite of sturgeons, became sexually mature in this ancient fish when the ichthyosaur that once supported its final stage became extinct. Other species of Amphilina can attain sexual maturity in their first crustacean host (Stunkard, 1962, p. 28). Finally, the pseudophyllidean genus Bothrimonus often reaches sexual maturity in its single intermediate host, usually a brackish-water gammarid.
I have already discussed the progenesis of Polystomum, a trematode. Polystomum integerrimum usually matures in the bladder of a frog after initial attachment to the gills of its tadpole; there it becomes sexually mature after three years (Caullery, 1952). But if a Polystomum attaches to a sufficiently young tadpole, it grows rapidly on the gills and reproduces there as a progenetic form. The normal parasites are far more fecund (Williams, 1961), but the progenetics can pass through several generations before the normals mature. Some species that parasitize fully aquatic amphibians have become permanently progenetic. The closely related Sphyranum lives on the gills of perennibranchiate Necturus (Williams, 1961). Protopolystoma xenopi infests the aquatic frog Xenopus laevis (Williams, 1961), where it matures in the bladder (Xenopus is a normal frog with no external gills). Infection may be direct (through the cloacal opening) and the mature trematode is strongly paedomorphic (organs last to differentiate in normal ontogeny are the first to disappear in these progenetics).
Ghiselin (1974) has identified and presented impressive documentation for a common reproductive strategy among males in many small, marine invertebrates. These males are progenetic dwarfs, often minute compared with their females (Fig. 56). Ghiselin identifies the following conditions as prerequisites for this variety of sexual selec-