66
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
host cellular reactions. However, in the four species of unnatural
hosts into which specific miracidia successfully penetrated, severe
host-tissue reactions occurred in certain host-parasite associations.
These were in the form of encapsulation by cells and fibers. I n others,
no cellular responses were apparent but the parasite did not develop
normally, eventually dying, thus suggesting the occurrence of some
type of innate humoral response.
Mackin (1961), in an abstract, has reported that the entire encapsulation complex, i.e. host cells and foreign material, in oysters
may be discharged through the external epithelium. He did not present
any experimental data for this, and until convincing evidence becomes
available, this type of mass elimination must be doubted. I n the same
abstract, Mackin has reported that leucocytes which contribute to
encapsulation will later form epithelial tissue surrounding the zone of
infection. Again no experimental evidence was provided, and since
this concept appears to be in direct contradiction to the evidences of
others cited below, it cannot be accepted a t this time.
The works of Newton, Brooks, and Sudds mentioned above wouid
suggest the complete absence of the encapsulation process surrounding
larval trematodes in their natural hosts. This is true if conspicuous
massive encapsulations are sought for ; however, Cheng and Cooperman
(1964) have demonstrated that if Helisoma trivolvis, the natural host,
becomes heavily infected with Glypthelmins pennsylvaniensis larvae,
sporocysts and cercariae escaping from the hepatopancreas will invade
and continue to develop in the host’s reproductive system and auxiliary
glands. When found in the albumin gland, encapsulation will occur
although the capsule is generally weak, being comprised of a thin layer
of connective tissue fibers. Similarly, escaping cercariae trapped in the
snail’s foot musculature will become encapsulated by myofibers and
connective tissue fibers. I n neither instance, however, are the parasites
killed. The lack of host cellular responses elsewhere in the host’s body
caused Cheng and Cooperman to state :
From the instances of encapsulation observed, it is now evident that only
certain types of host tissues are capable of encapsulation. In this instance,
encapsulation occurs only when connective tissue and myofibers are present.
This observation is upheld by the fact that host-elaborated envelopes
surrounding the larvae of G. pennsylvaniensis are only in the albumin gland,
which possesses a connective tissue tunica, and in the foot, which includes
muscle fibers and connective tissues.
This concept has been strengthened by the results of Cheng et al.
( 1966a) who experimentally introduced known numbers of Himasthla
quissetensis cercariae into eight species of marine pelecypods and
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
host cellular reactions. However, in the four species of unnatural
hosts into which specific miracidia successfully penetrated, severe
host-tissue reactions occurred in certain host-parasite associations.
These were in the form of encapsulation by cells and fibers. I n others,
no cellular responses were apparent but the parasite did not develop
normally, eventually dying, thus suggesting the occurrence of some
type of innate humoral response.
Mackin (1961), in an abstract, has reported that the entire encapsulation complex, i.e. host cells and foreign material, in oysters
may be discharged through the external epithelium. He did not present
any experimental data for this, and until convincing evidence becomes
available, this type of mass elimination must be doubted. I n the same
abstract, Mackin has reported that leucocytes which contribute to
encapsulation will later form epithelial tissue surrounding the zone of
infection. Again no experimental evidence was provided, and since
this concept appears to be in direct contradiction to the evidences of
others cited below, it cannot be accepted a t this time.
The works of Newton, Brooks, and Sudds mentioned above wouid
suggest the complete absence of the encapsulation process surrounding
larval trematodes in their natural hosts. This is true if conspicuous
massive encapsulations are sought for ; however, Cheng and Cooperman
(1964) have demonstrated that if Helisoma trivolvis, the natural host,
becomes heavily infected with Glypthelmins pennsylvaniensis larvae,
sporocysts and cercariae escaping from the hepatopancreas will invade
and continue to develop in the host’s reproductive system and auxiliary
glands. When found in the albumin gland, encapsulation will occur
although the capsule is generally weak, being comprised of a thin layer
of connective tissue fibers. Similarly, escaping cercariae trapped in the
snail’s foot musculature will become encapsulated by myofibers and
connective tissue fibers. I n neither instance, however, are the parasites
killed. The lack of host cellular responses elsewhere in the host’s body
caused Cheng and Cooperman to state :
From the instances of encapsulation observed, it is now evident that only
certain types of host tissues are capable of encapsulation. In this instance,
encapsulation occurs only when connective tissue and myofibers are present.
This observation is upheld by the fact that host-elaborated envelopes
surrounding the larvae of G. pennsylvaniensis are only in the albumin gland,
which possesses a connective tissue tunica, and in the foot, which includes
muscle fibers and connective tissues.
This concept has been strengthened by the results of Cheng et al.
( 1966a) who experimentally introduced known numbers of Himasthla
quissetensis cercariae into eight species of marine pelecypods and