nswers to
the dorsal or ventral surface of a gasteropod. It is not decided
whether the arms and funnels of the one have or have not their
homologues in the other. The dorsal integument of a Doris and the
cloak of a whelk are both called 'mantle,' without any evidence
to show that they are really homologous. Nor do very much more
definite notions seem to have prevailed with regard to the
archetypal molluscous form, and the mode in which (if such an
archetype exist) it becomes modified in the different secondary
types."

He had taken from the surface of the sea a number of transparent
shell-fish, and had been able to study the structure and arrangement
of their organs "by simple inspection, without so much as disturbing a
single beat of their hearts." From knowledge gained in this fashion,
and from ordinary dissection of a number of common snails,
cephalopods, and pteropods, he was able to describe in a very complete
way the anatomical structure of cephalous molluscs. The next natural
step, he stated, would have been to describe the embryonic development
of the organs of these different creatures in order that a true
knowledge might be gained of what were the homologous or really
corresponding parts in each. Having had no opportunity to make such
embryological studies for himself, he fell back on numerous accounts
of development by Koelliker, Van Beneden, Gegenbauer, and others, and
so gradually arrived at a conception of what he called the "archetype"
of the cephalous molluscs. As the word _archetype_ was borrowed from
old metaphysical ideas dating back to the time of Plato, he took care
to state that what he meant by it was no more than a form embodying
all that could be affirmed equally respecting every single kind of
cephalous mollusc, and by no means an "idea" upon which it could be
supposed that animal forms had been modelled. He described this
archetype, and showed the condition of the different systems of organs
which it could be supposed to possess, and how these organs were
modified in the different existing groups. This archetypal mollusc of
Huxley's was a creature with a bilaterally symmetrical head and body.
On the ventral side of the body it possessed a peculiar locomotor
appendage, the so-called foot, and the dorsal surface of the body
secreted a shell. Its nervous system consisted of three pairs of
ganglia or brains, one pair in the head, one in the foot, and a