4. ANALYSIS OF FACTORS INVOLVED I N SYMBIOSIS
123
flex reversal. Also related to invasion of the molluscan hosts’
gonads by parasites is the interesting phenomenon of sex reversal.
Pelseneer (1906, 1928) appears to have been the first to point out that
the penes of infected snails are reduced, due, in his opinion, to the
partial or complete castration of the host. It was Wesenburg-Lund
(1931), however, who first proposed the theory that changes in sex in
molluscs might be directly due to the effect of larval trematodes. His
belief is based on his observations on the hermaphroditic snail Succinea
putris in which, after the female organs are completely destroyed, the
production of spermatozoa occurs for some time before complete castration. As the result of his observation, he stated : “ With some right
it may be maintained that many parasitized snails pass through a male
stage, or in other words that the parasite alters a hermaphrodite
organism into a male.”
Wesenberg-Lund’s report was followed by that of Krull (1935)
who studied the morphology of a number of snails, including Hydrobia
ulvae. This German investigator has reported that a reduced penis is
present in parasitized H . ulvae which actually represent “ sexreversed ’’ females. He believed that these were originally females,
since during the examination of internal organs which were only
partially destroyed he found what he considered to be unquestionably
female structures. The small penis was non-functional although there
is no question that it had developed in these females. Krull did not
suggest that these snails had ever functioned as males; therefore, as
Rothschild (1938) has pointed out, ‘‘ sex-reversal ” is an unsatisfactory
term in this instance.
W. J. Rees (1936a), as the result of his study on the effects of
parasitism on Littorina littorea, has reported that there are definite
reductions in the size and prominence of the accessory sex organs (penis
and vas deferens in males, oviduct in females) resulting from invasion
of the gonads by parasites. This appears to indicate that the sizes of
these accessory sex organs are dependent upon gonadal activity. The
dependence of the development of accessory reproductive organs in
gastropods has since been experimentally shown to be indeed dependent
upon a gonadal hormone (see review by Charniaux-Cotton and
Kleinholz, 1964). Rees has found that if the snails are parasitized by
either Himasthla leptosorna or Cercaria lophocerca, both of which completely destroy the host’s gonads, it is very difficult, if not impossible,
to determine their sex since the accessory organs become invisible. In
Littorina littorea parasitized by C. emasculans, the accessory glands are
reduced but are still visible. According to W. J. Rees, this is because
C. emasculans never completely destroys the host’s gonads. Although
123
flex reversal. Also related to invasion of the molluscan hosts’
gonads by parasites is the interesting phenomenon of sex reversal.
Pelseneer (1906, 1928) appears to have been the first to point out that
the penes of infected snails are reduced, due, in his opinion, to the
partial or complete castration of the host. It was Wesenburg-Lund
(1931), however, who first proposed the theory that changes in sex in
molluscs might be directly due to the effect of larval trematodes. His
belief is based on his observations on the hermaphroditic snail Succinea
putris in which, after the female organs are completely destroyed, the
production of spermatozoa occurs for some time before complete castration. As the result of his observation, he stated : “ With some right
it may be maintained that many parasitized snails pass through a male
stage, or in other words that the parasite alters a hermaphrodite
organism into a male.”
Wesenberg-Lund’s report was followed by that of Krull (1935)
who studied the morphology of a number of snails, including Hydrobia
ulvae. This German investigator has reported that a reduced penis is
present in parasitized H . ulvae which actually represent “ sexreversed ’’ females. He believed that these were originally females,
since during the examination of internal organs which were only
partially destroyed he found what he considered to be unquestionably
female structures. The small penis was non-functional although there
is no question that it had developed in these females. Krull did not
suggest that these snails had ever functioned as males; therefore, as
Rothschild (1938) has pointed out, ‘‘ sex-reversal ” is an unsatisfactory
term in this instance.
W. J. Rees (1936a), as the result of his study on the effects of
parasitism on Littorina littorea, has reported that there are definite
reductions in the size and prominence of the accessory sex organs (penis
and vas deferens in males, oviduct in females) resulting from invasion
of the gonads by parasites. This appears to indicate that the sizes of
these accessory sex organs are dependent upon gonadal activity. The
dependence of the development of accessory reproductive organs in
gastropods has since been experimentally shown to be indeed dependent
upon a gonadal hormone (see review by Charniaux-Cotton and
Kleinholz, 1964). Rees has found that if the snails are parasitized by
either Himasthla leptosorna or Cercaria lophocerca, both of which completely destroy the host’s gonads, it is very difficult, if not impossible,
to determine their sex since the accessory organs become invisible. In
Littorina littorea parasitized by C. emasculans, the accessory glands are
reduced but are still visible. According to W. J. Rees, this is because
C. emasculans never completely destroys the host’s gonads. Although