. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 72 CHAUNCEY G. GOODCHILD cercariae, on the other hand, lack precociously developed testes [except C. Gorgodera cygnoides, C. Gorgodera pagenstecheri, C. Gorgodera varsoviensis and C. Gorgoderina vitelliloba described by Sinitsin (1905)], possess penetration and cystogenous gland cells, usually lack a postacetabularly flared body, and parasitize members of the family Sphaeriidae; stylet cercariae are eaten by second inter- mediate vector hosts in which they encyst to form infective metacercariae; adults from stylet gorgoderid
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 72 CHAUNCEY G. GOODCHILD cercariae, on the other hand, lack precociously developed testes [except C. Gorgodera cygnoides, C. Gorgodera pagenstecheri, C. Gorgodera varsoviensis and C. Gorgoderina vitelliloba described by Sinitsin (1905)], possess penetration and cystogenous gland cells, usually lack a postacetabularly flared body, and parasitize members of the family Sphaeriidae; stylet cercariae are eaten by second inter- mediate vector hosts in which they encyst to form infective metacercariae; adults from stylet gorgoderid cercariae are parasites of fishes and amphibians. All sexually mature phyllodistomes have a similar morphological pattern. Upon the basis of adult structures, it is impossible to determine the cercarial type of any adult whose life-history is unknown. If subsequent investigation proves that the adults, because of similar habitats, have converged to a common morphological form from diverse ancestry (Text Figure 1) it will be a most ??7 (Catoptroldes) Phyllodlstomum Gorgoderlna Gorgodera. Cercariae without Cercariae with Cercariae with stylets stylets; stylets; motile non-motile Unlonld and Drelas- Sphaerlld parasites. enld parasites. FIGURE 1. Assuming convergent evolution of adults from diverse cercarial ancestry. striking case of convergent evolution; if the present concept of close adult relation- ships holds, however, which seems most likely, it must be assumed then that cercarial structures in the gorgoderids, at least, are only caenogenetic features which have been modified to fit the exigencies of their varied life-histories (Text Figure 2). 5. Metacercaria. One hundred and forty-four damsel-fly naiads were each experimentally infected with from one to ten metacercariae. The latter are easily seen in vivo with intense reflected light because the opaque concretions in the excretory bladder appear pearly white. The metacercarial cysts (Figure 8) are spherical to ellipsoida
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