2. STEROLS: STRUCTURE AND DISTRIBUTION
137
and Stenoglossa have been investigated to show convincingly that
cholesterol is the major component of their sterol mixtures (see Table
VII).
The situation is not as clear for species from the subclass of
Ophisthobranchia. One species each from the orders Pleurocoela (Tectibranchia) and Acoela (Nudibranchia) have afforded unusual sterol
mixtures with a single ultraviolet absorption peak at 277-278 τημ. From
two species of the order Pteropoda, sterol mixtures have been obtained
in which the major component is a substance tentatively called pteropodasterol, acetate, m.p. 136° (135), rather than cholesterol. This differentiation from other Gastropoda is of considerable interest. The
pelagic, free-swimming snails belonging to the Pteropoda had long
been considered by zoologists to belong to a class equivalent in value
to the Gastropoda, and had even been regarded as the ancestors of the
Cephalopoda. The unusual sterol sets these snails indeed apart, at least
from other species of the subclass of Prosobranchia. It does not, however, point to a relationship to the Cephalopoda, which contain cholesterol as the nearly exclusive component of their sterol mixture.
The data available for sterols of species from the subclass of Pulmonata, the land and freshwater snails, are too inadequate to permit even
preliminary generalizations. It appears, however, that they also contain
cholesterol as the major sterol, accompanied, however, by rather large
quantities of A
57 -sterols. Considerable quantities of such provitamins D
have also been found in marine Gastropoda (see Table VII). The
familiar whelk, Buccinum undatum, for example, has been shown to
contain 78% of 7-dehydrocholesterol (I, R = A) in its sterol mixture
(106).
The sterol mixtures of the Pelecypoda, the bivalves, are of particular
interest. They also contain cholesterol (136), but in smaller quantities
than the mixtures obtainable from many of the Gastropoda. The other
sterols, complex a mixture as they may be, afford acetates that, even
when crude, have a melting point significantly higher than cholesteryl
acetate, m.p. 115-116°. A mollusk sterol acetate mixture melting between 110° and 120° points toward cholesteryl acetate as its major component and hence to Gastropoda as its source material. A melting point
above 130°, however, indicates the predominant presence of other
steryl acetates and Pelecypoda as the source material. It should be
borne in mind, however, that these differences are subject to seasonal
variations, and that exceptions have been noted as in the case of the
Pteropoda. Nevertheless such figures are of help in the analysis of the
data given in Table VII.
In 1934 Bergmann (137) and Tsujimoto and Koyanagi (138) in-
137
and Stenoglossa have been investigated to show convincingly that
cholesterol is the major component of their sterol mixtures (see Table
VII).
The situation is not as clear for species from the subclass of
Ophisthobranchia. One species each from the orders Pleurocoela (Tectibranchia) and Acoela (Nudibranchia) have afforded unusual sterol
mixtures with a single ultraviolet absorption peak at 277-278 τημ. From
two species of the order Pteropoda, sterol mixtures have been obtained
in which the major component is a substance tentatively called pteropodasterol, acetate, m.p. 136° (135), rather than cholesterol. This differentiation from other Gastropoda is of considerable interest. The
pelagic, free-swimming snails belonging to the Pteropoda had long
been considered by zoologists to belong to a class equivalent in value
to the Gastropoda, and had even been regarded as the ancestors of the
Cephalopoda. The unusual sterol sets these snails indeed apart, at least
from other species of the subclass of Prosobranchia. It does not, however, point to a relationship to the Cephalopoda, which contain cholesterol as the nearly exclusive component of their sterol mixture.
The data available for sterols of species from the subclass of Pulmonata, the land and freshwater snails, are too inadequate to permit even
preliminary generalizations. It appears, however, that they also contain
cholesterol as the major sterol, accompanied, however, by rather large
quantities of A
57 -sterols. Considerable quantities of such provitamins D
have also been found in marine Gastropoda (see Table VII). The
familiar whelk, Buccinum undatum, for example, has been shown to
contain 78% of 7-dehydrocholesterol (I, R = A) in its sterol mixture
(106).
The sterol mixtures of the Pelecypoda, the bivalves, are of particular
interest. They also contain cholesterol (136), but in smaller quantities
than the mixtures obtainable from many of the Gastropoda. The other
sterols, complex a mixture as they may be, afford acetates that, even
when crude, have a melting point significantly higher than cholesteryl
acetate, m.p. 115-116°. A mollusk sterol acetate mixture melting between 110° and 120° points toward cholesteryl acetate as its major component and hence to Gastropoda as its source material. A melting point
above 130°, however, indicates the predominant presence of other
steryl acetates and Pelecypoda as the source material. It should be
borne in mind, however, that these differences are subject to seasonal
variations, and that exceptions have been noted as in the case of the
Pteropoda. Nevertheless such figures are of help in the analysis of the
data given in Table VII.
In 1934 Bergmann (137) and Tsujimoto and Koyanagi (138) in-