The origin of the cnidarians and
ctenophores is obscure, although the
most widely supported hypothesis
today is that the radiate phyla arose
from a radially symmetrical, planulalike
ancestor. Such an ancestor could
have been common to the radiates and to the higher metazoans, the latter having
been derived from a branch whose
members habitually crept about on the
sea bottom. Such a habit would select
for bilateral symmetry. Others became
sessile or free floating, conditions for
which radial symmetry is a selective
advantage. A planula larva in which an
invagination formed to become the
gastrovascular cavity would correspond
roughly to a cnidarian with an
ectoderm and an endoderm.
Some researchers believe trachyline
medusae (an order of class Hydrozoa)
resemble the ancestral cnidarian
because of their direct development
from planula and actinula larvae to
medusas (Figure 13-38). The trachylinelike
ancestor would have given rise to
other cnidarian lines after the evolution
of the polyp stage and alternation of
sexual (medusa) and asexual (polyp)
generations. Subsequently, the medusa
was completely lost in the anthozoan
line. If the order Trachylina is retained
within class Hydrozoa, however, then
Hydrozoa becomes paraphyletic. Future
investigators may resolve this problem.
Cladogram showing hypothetical relationships of cnidarian classes with some shared derived characters indicated.
This hypothesis suggests that the
hydrozoan order Trachylina retains the ancestral cnidarian life cycle, having
branched off before the evolution of the polyp stage. Note that this arrangement
makes the Hydrozoa paraphyletic;
the trachyline-like Hydrozoa is the sister group to all the other Cnidaria.
Ctenophores formerly were thought
to have arisen from a medusoid cnidarian,
but this hypothesis has been challenged.
Similarities between the
groups are mostly of a general nature
and do not seem to indicate a close
relationship. Molecular evidence suggests
that ctenophores branched from
the metazoan line after sponges but
before cnidarians and placozoans.
In their evolution neither phylum has
deviated far from its basic plan of structure.
In Cnidaria, both polyp and
medusa are constructed on the same scheme. Likewise, ctenophores have
adhered to the arrangement of the comb
plates and their biradial symmetry.
Nonetheless, cnidarians have
achieved large numbers of individuals
and species, demonstrating a surprising
degree of diversity considering the
simplicity of their basic body plan.
They are efficient predators, many
feeding on prey quite large in relation
to themselves. Some are adapted for
feeding on small particles. The colonial
form of life is well explored, with
some colonies growing to great size
among corals, and others, such as
siphonophores, showing astonishing
polymorphism and specialization of
individuals within a colony.