134
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
active process, with the cercariae utilizing glandular secretions combined with mechanical activity. Richards (1961) has confirmed Duke’s
observations.
The escape mechanism among non-schistosome ceroariae is known
only for three species. Kendall and McCullough (1951), working with
Fasciola hepatica cercariae, have reported that these travel to the site of
emergence, which is in the region contiguous with the snail’s anus,
and that emergence is a passive (involuntary) process, at least in the
later stages. The force behind the ejection is in the form of pressures
set up in the host’s mantle cavity which in turn cause pressures to be
exerted on the perivisceral spaces containing the cercariae. Pearson
(1961), working with Neodiplostomum intermedium cercariae in Pettancylus assirnilis, has observed cercariae free within the visceral haemocoel
and has stated that “ cercariae crawled about the visceral hnemocoel,
probing vigorously with the anterior end, and entered the first part
of the peri-rectal sinus and mantle vessels ”. He has further observed
that cercariae escape from the mantle via a fixed “escape pore ”.
According to him, a similar phenomenon has been observed in seven
other species of strigeid cercariae.
Finally, Probert and Erasmus (1965) have contributed a detailed
study of the migration and escape of the strigeid cercaria, Cercaria X,
from Lymnaea stagnalis. By studying serial sections of infected snails,
they have found that the main pathway along which migrating cercariae
travel is the blood vascular system. The cercariae enter the main
visceral haemocoel after the tunica propria of the host’s hepatopancreas ruptures, and from there enter the main venous vessels,
deploying through the rectal sinus, sub-renal sinus, lung, and heart.
The cercariae then travel from the heart via the mantle arteries and
become localized in the mantle sinuses prior to emergence. These
investigators are of the opinion that while in the circulatory system
the cercariae are partially active. The inner surface of the mantle,
particularly the leading edge, is said to be the main site of escape.
Unlike the cercariae of F . hepatica as has been reported by Kendall
and McCullough (1951), Probert and Erasmus believe that the final
escape of Cercaria X is an active (voluntary) process.
From the above, it would appear that cercariae can escape from
their molluscan hosts via different routes and can be either actively
motile, passively carried along in the blood, or pushed by pressures.
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
active process, with the cercariae utilizing glandular secretions combined with mechanical activity. Richards (1961) has confirmed Duke’s
observations.
The escape mechanism among non-schistosome ceroariae is known
only for three species. Kendall and McCullough (1951), working with
Fasciola hepatica cercariae, have reported that these travel to the site of
emergence, which is in the region contiguous with the snail’s anus,
and that emergence is a passive (involuntary) process, at least in the
later stages. The force behind the ejection is in the form of pressures
set up in the host’s mantle cavity which in turn cause pressures to be
exerted on the perivisceral spaces containing the cercariae. Pearson
(1961), working with Neodiplostomum intermedium cercariae in Pettancylus assirnilis, has observed cercariae free within the visceral haemocoel
and has stated that “ cercariae crawled about the visceral hnemocoel,
probing vigorously with the anterior end, and entered the first part
of the peri-rectal sinus and mantle vessels ”. He has further observed
that cercariae escape from the mantle via a fixed “escape pore ”.
According to him, a similar phenomenon has been observed in seven
other species of strigeid cercariae.
Finally, Probert and Erasmus (1965) have contributed a detailed
study of the migration and escape of the strigeid cercaria, Cercaria X,
from Lymnaea stagnalis. By studying serial sections of infected snails,
they have found that the main pathway along which migrating cercariae
travel is the blood vascular system. The cercariae enter the main
visceral haemocoel after the tunica propria of the host’s hepatopancreas ruptures, and from there enter the main venous vessels,
deploying through the rectal sinus, sub-renal sinus, lung, and heart.
The cercariae then travel from the heart via the mantle arteries and
become localized in the mantle sinuses prior to emergence. These
investigators are of the opinion that while in the circulatory system
the cercariae are partially active. The inner surface of the mantle,
particularly the leading edge, is said to be the main site of escape.
Unlike the cercariae of F . hepatica as has been reported by Kendall
and McCullough (1951), Probert and Erasmus believe that the final
escape of Cercaria X is an active (voluntary) process.
From the above, it would appear that cercariae can escape from
their molluscan hosts via different routes and can be either actively
motile, passively carried along in the blood, or pushed by pressures.