60
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
uninvestigated among parasitized marine molluscs, studies on freshwater gastropods parasitized by larval trematodes suggest that such
hormones may influence the growth and differentiation of the intramolluscan trematode larvae (Szidat, 1959 ; Meade and Pratt, 1966).
2. Internal defense mechanisms
Our understanding of internal defense mechanisms of molluscs is a t
a relatively primitive state as compared to what is known about insect
defense mechanisms among the invertebrates. This is particularly true
in the case of humoral factors (see reviews by Huff, 1940 ; Bisset, 1947 ;
Stauber, 1961 ; Cheng and Sanders, 1962 ; Tripp, 1963). It is not surprising that much of what is known about defense mechanisms in
molluscs has come from studies on commercially important marine
species, especially the American oyster, Crassostrea virginica. As the
result of the late Dr Thurlow C. Nelson's interest in oyster research a t
Rutgers University, many of his students have continued this line of
investigation. Among these, Dr Leslie A. Stauber, Dr Harold H. Haskin
and their students have pioneered in studying defense mechanisms in
C. virginica, especially the role of leucocytes.
In order t o present a more complete picture of defense mechanisms
in molluscs, it is necessary that pieces of information gained from
investigations on fi-eshwater species be interjected where appropriate to
suggest some specific mechanism or improve upon our understanding of
the total picture. Such may in addition serve to stimulate comparable
studies on marine species. Furthermore, in order to facilitate the
presentation, molluscan defense mechanisms, in following the general
pattern of Stauber (1961) and Cheng and Sanders (1962), have been
divided into subcategories, namely, phagocytosis, encapsulation,
leucocytosis, nacrezation, and humoral factors.
a. Phagocytosis
The role of phagocytes in defense reactions of invertebrates and
vertebrates has been recognized ever since the original studies of
Metchnikoff and others (Metchnikoff, 1901). Actually Haeckel (1862)
was the first to discover phagocytosis. He injected particulate dyes into
molluscs so that the distribution pattern of their vascular systems might
be determined. I n so doing, he accidentally discovered that leucocytes"
phagocytized the dyes. Following these studies were those of De
* Since the various types of leucocytes found in the majority, if not all, molluscs
remain uncertain, the terms " leucocyte ", '' amoebocyte " and " phagocyte " me
used interchangeably in this paper. They all scrve to designate the free leucocytes in
molluscs which are amoeboid and are capable of phagocytosis.
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
uninvestigated among parasitized marine molluscs, studies on freshwater gastropods parasitized by larval trematodes suggest that such
hormones may influence the growth and differentiation of the intramolluscan trematode larvae (Szidat, 1959 ; Meade and Pratt, 1966).
2. Internal defense mechanisms
Our understanding of internal defense mechanisms of molluscs is a t
a relatively primitive state as compared to what is known about insect
defense mechanisms among the invertebrates. This is particularly true
in the case of humoral factors (see reviews by Huff, 1940 ; Bisset, 1947 ;
Stauber, 1961 ; Cheng and Sanders, 1962 ; Tripp, 1963). It is not surprising that much of what is known about defense mechanisms in
molluscs has come from studies on commercially important marine
species, especially the American oyster, Crassostrea virginica. As the
result of the late Dr Thurlow C. Nelson's interest in oyster research a t
Rutgers University, many of his students have continued this line of
investigation. Among these, Dr Leslie A. Stauber, Dr Harold H. Haskin
and their students have pioneered in studying defense mechanisms in
C. virginica, especially the role of leucocytes.
In order t o present a more complete picture of defense mechanisms
in molluscs, it is necessary that pieces of information gained from
investigations on fi-eshwater species be interjected where appropriate to
suggest some specific mechanism or improve upon our understanding of
the total picture. Such may in addition serve to stimulate comparable
studies on marine species. Furthermore, in order to facilitate the
presentation, molluscan defense mechanisms, in following the general
pattern of Stauber (1961) and Cheng and Sanders (1962), have been
divided into subcategories, namely, phagocytosis, encapsulation,
leucocytosis, nacrezation, and humoral factors.
a. Phagocytosis
The role of phagocytes in defense reactions of invertebrates and
vertebrates has been recognized ever since the original studies of
Metchnikoff and others (Metchnikoff, 1901). Actually Haeckel (1862)
was the first to discover phagocytosis. He injected particulate dyes into
molluscs so that the distribution pattern of their vascular systems might
be determined. I n so doing, he accidentally discovered that leucocytes"
phagocytized the dyes. Following these studies were those of De
* Since the various types of leucocytes found in the majority, if not all, molluscs
remain uncertain, the terms " leucocyte ", '' amoebocyte " and " phagocyte " me
used interchangeably in this paper. They all scrve to designate the free leucocytes in
molluscs which are amoeboid and are capable of phagocytosis.