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218 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES
Series 4, Volume 60, No. 10
1985; Mori, Takikawa, Kido Albizati, & Faulkner, 1992; Pawlik, Albizati & Faulkner, 1986).
Superficially similar triterpenoids, testudinariols A and B (Atlas 515, 516), occur in an opisthobranch,
Pleurobranchus testudinarius (Photo 43) (Spinella, Mollo, Trivellone & Cimino, 1997).
Testudinariol B also occurs in an as yet unidentified species of Pleurobranchus from China
(Carbone, 2007). The metabolites in both the prosobranch and the opisthobranchs exhibit a
squalane skeleton that apparently is a dimer, formed by the union of a pair of farnesyl sesquiterpenoid
moieties.
We begin our account of Euthyneura by an effort to reconstruct the features on the common
ancestor of opisthobranchs and pulmonates. This has been attempted by several authors, mainly on
the basis of comparing them to more distantly related animals and trying to decide what the primitive
conditions are on a physiological basis (Ghiselin, 1966b; Gosliner, 1981). One criterion that
needs to be avoided is that of treating such “primitive” animals as the cephalaspidean opisthobranch
Acteon, and especially its best-known representative, A. tornatilis, as if it were the common
ancestor. Although primitive in many respects, such as having a rather well developed shell and
less of the detorsion that is characteristic of opisthobranchs, Acteon has some derived characters,
most notably a carnivorous diet and a reproductive system with a separate vas deferens instead of
an open, ciliated groove. Presence (primitive) or absence (derived) of an operculum in the adult
varies within the family Acteonidae. A. tornatilis has a much modified radula, assuming that what
has been called a radula is not something else (Gabe & Prenant, 1953).
The common ancestor had a shell, well enough developed that the animal could withdraw into
it, supplemented by an operculum. The nervous system did not display many of the effects of detorsion
and concentration of ganglia that characterize the more derived representatives of the group.
The animal fed mainly on plant material, perhaps including some detritus. It had a relatively unspecialized
radula, and its gut possessed at most some cuticularizations rather than the gizzard plates
that characterize some of the more derived forms. We may speculate that some reduction of the
shell had already begun, perhaps as a consequence of feeding on chemically protected food organisms.
A benthic habitat with a significant amount of sediment can be justified on the basis of the
changes in the gills and their surrounding (mantle) cavity that can be documented in both opisthobranchs
and pulmonates. The mechanisms causing movement of water through the cavity and over
the gill have changed, and in correlation so too has the structure of the gill. Instead of the respiratory
current being produced mainly by cilia that cover the gill surface, water is moved by the action
of bands of cilia behind the gill, and the gill itself presents a series of folds, or plicae, rather than
comb-like structures to the water that flows over it (in other words there was a plicate rather than
a pectinate gill).
The ancestor was a simultaneous hermaphrodite with an undivided gonoduct, organs that
stored and processed spermatozoa, and a series of regions that deposited three layers of material
(albumen, membrane, and mucus) around them to form an egg mass. From the common opening
there was a ciliated groove that conveyed sperm to the penis, which was located at the anterior end
of the body. The life cycle included a free-swimming larva that emerged from a protective egg
mass.
Some authors have advocated alternative views about the ancestral state of the gills and the
male portion of the reproductive system. So far as the gills go, the “evidence” is merely that the
gills are different, not that there is anything implausible about the derivation. The claim that the
male part of the system has evolved from a closed duct into an open groove is obviously an attempt
to justify physiologically implausible results instead of asking what went wrong with the analysis.
Molecular as well as anatomical evidence indicates that both Opisthobranchia and Pulmonata
are monophyletic lineages. The possibility that one opisthobranch lineage or another is closer to