4. ANALYSIS OF FACTORS INVOLVED IN SYMBIOSIS
57
Read and Simmons, 1963) or that of vertebrate hosts on the molting of
nematodes (Rogers, 1960, 1962) remain unexplored.
There is some evidence that molluscs do provide some type of
stimulus which causes the shedding of trematode miracidial epidermal
plates. Campbell and Todd (1955b), for example, have reported that
the miracidium of Pascioloides magna will shed its ciliated epidermal
plates after a short contact with tissues of its molluscan host and will
metamorphose into a sporocyst in vitro. As Dawes (1960) has pointed
out, other trematode miracidia also shed their ciliated epidermis prior
t o penetration of their molluscan hosts, and it is possible that it is the
stimulus contributed by the mollusc which initiates the shedding
process. It should be pointed out, however, that certain species of
trematodes, such as Schistosoma mansoni, apparently do not shed their
ciliated epidermis prior to penetration. Maldonado and Matienzo
(1947) have demonstrated that, in the case of S. mansoni, shedding of
the plates does not occur until the miracidium has penetrated the
snail host.
According to Lengy (1962), discharge of the contents of the penetration glands and apical gland of Schistosoma bovis miracidia is also
stimulated by contact with the molluscan host. Similarly Cheng (1967)
has demonstrated that when Fasciola gigantica miracidia are exposed
t o the plasma of Galba ollula, the natural molluscan host in Hawaii, and
Helisoma duryi normale, an incompatible host, not only is the secretion
of some substance, perhaps the lytic enzyme, observed, but the apical
papilla of many of the miracidia also becomes invaginated to form a
terminal cup. This finding suggests that the stimulatory factor(s) is not
limited to the plasma of compatible hosts. Although the chemical
nature of the stimulants contributed by the host which elicit shedding
or glandular secretion is not yet known, these fragmentary pieces of
information suggest that molluscs do contribute morphogenetic factors.
It is also known that the plasma of certain molluscs will enhance
the establishment of certain trematode larvae (Cheng, 1963a). Although
in this instance the sera (plasma or hemolymph) do not bring about
morphogenetic changes, they do serve to activate infectj ve cercariae.
Specifically, it has been shown that the sera (plasma or hemolymph) of
five species of freshwater molluscs, Viviparus malleatus, Helisoma
trivolvis, H . anceps, Physa gyrina and Musculium partumeium, will
activate the normally quiescent cystocercous cercaria of Gorgodera
amplicava. The activation has been shown not t o be due to the pH of
the plasma but to some component fraction. I n nature, once the
quiescent cercaria becomes ingested by the molluscan second intermediate host, it is believed that the plasma seeping into the mollusc’s
57
Read and Simmons, 1963) or that of vertebrate hosts on the molting of
nematodes (Rogers, 1960, 1962) remain unexplored.
There is some evidence that molluscs do provide some type of
stimulus which causes the shedding of trematode miracidial epidermal
plates. Campbell and Todd (1955b), for example, have reported that
the miracidium of Pascioloides magna will shed its ciliated epidermal
plates after a short contact with tissues of its molluscan host and will
metamorphose into a sporocyst in vitro. As Dawes (1960) has pointed
out, other trematode miracidia also shed their ciliated epidermis prior
t o penetration of their molluscan hosts, and it is possible that it is the
stimulus contributed by the mollusc which initiates the shedding
process. It should be pointed out, however, that certain species of
trematodes, such as Schistosoma mansoni, apparently do not shed their
ciliated epidermis prior to penetration. Maldonado and Matienzo
(1947) have demonstrated that, in the case of S. mansoni, shedding of
the plates does not occur until the miracidium has penetrated the
snail host.
According to Lengy (1962), discharge of the contents of the penetration glands and apical gland of Schistosoma bovis miracidia is also
stimulated by contact with the molluscan host. Similarly Cheng (1967)
has demonstrated that when Fasciola gigantica miracidia are exposed
t o the plasma of Galba ollula, the natural molluscan host in Hawaii, and
Helisoma duryi normale, an incompatible host, not only is the secretion
of some substance, perhaps the lytic enzyme, observed, but the apical
papilla of many of the miracidia also becomes invaginated to form a
terminal cup. This finding suggests that the stimulatory factor(s) is not
limited to the plasma of compatible hosts. Although the chemical
nature of the stimulants contributed by the host which elicit shedding
or glandular secretion is not yet known, these fragmentary pieces of
information suggest that molluscs do contribute morphogenetic factors.
It is also known that the plasma of certain molluscs will enhance
the establishment of certain trematode larvae (Cheng, 1963a). Although
in this instance the sera (plasma or hemolymph) do not bring about
morphogenetic changes, they do serve to activate infectj ve cercariae.
Specifically, it has been shown that the sera (plasma or hemolymph) of
five species of freshwater molluscs, Viviparus malleatus, Helisoma
trivolvis, H . anceps, Physa gyrina and Musculium partumeium, will
activate the normally quiescent cystocercous cercaria of Gorgodera
amplicava. The activation has been shown not t o be due to the pH of
the plasma but to some component fraction. I n nature, once the
quiescent cercaria becomes ingested by the molluscan second intermediate host, it is believed that the plasma seeping into the mollusc’s