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196 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES
Series 4, Volume 60, No. 10
namesakes are analogous, not homologous), the term is no longer used. However, like the vertebrate
liver, it is an important site of detoxification.
The stomach and digestive gland of opisthobranchs have been convenient sources of interesting
metabolites for investigation. However, they do not provide very effective defense in that position,
and getting them to more appropriate locations is an important evolutionary theme. The
metabolites generally become concentrated in the skin of the animal, and in particular locations
where they will be most effective. Specialized “repugnatorial glands” are common, and the mantle
dermal formations already mentioned are a good example.
Sea slugs are rich in mucus, and may release copious quantities of it when they are attacked.
This mucus may contain a substantial amount of secondary metabolites, even when the basic effect
on the predator is a mechanical one. Mucus evidently represents the first line of chemical defense,
and the repugnatorial glands come into play when mucus is not sufficient to repel the predator and
the mollusk is bitten (Mollo, Gavagnin, Carbone, Guo & Cimino, 2005). Another place where secondary
metabolites may be concentrated is the gonad, which in opisthobranchs is called the “hermaphroditic
gland” because every animal is both male and female at the same time and produces
both sperm and eggs. The eggs are enveloped in protective mucus, and it is they (or the layer of
nutritive albumen surrounding them), that contain the metabolites.
CHAPTER II
COMPARISONS AND EXPERIMENTS
Part 1. Introduction
Epistemology is the branch of philosophy that is often defined as the theory of knowledge. It
considers the question “How do we know what is true?” Among scientists, practical epistemological
issues are often discussed under such rubrics as methodology and experimental design. When
scientists start writing polemics about methodology it usually is not a good sign, because it suggests
that they cannot find a straightforward means of settling their differences. In this chapter we
try to present a straightforward, yet philosophical, explanation of how research in the areas of interest
is carried out and why it is done that way. Scientific research is not easy, and we illustrate some
of the difficulties and pitfalls with examples drawn from our own research as well as that of others.
On a more positive note, there are also examples of ingenious experiments and clever arguments
from which valuable lessons can be learned.
Part 2. Systematics
Opisthobranchs of the order Anaspidea are called “sea-hares” because of their fanciful resemblance
to a bunny (Photo 17). The comparison has never fooled anybody. But it illustrates the point
that much more is involved in classification than just putting similar objects together. Scientists go
out of their way to avoid being deceived by appearances, and this is especially true of systematic
biologists, who often get burned by them. They need to sort their materials out into groups of
objects that correspond to objective reality, and that is not always easy. The problems are notoriously
difficult even at the most basic level of classification: the species. Here we will evade the
controversial aspects of what it means to be a species by referring the interested reader to the
inevitably long and difficult book that one of us has written on such topics (Ghiselin, 1997).
In evolutionary biology, a species is a reproductive community that is held together by sex.