The early embryological development
of sipunculans, echiurans, and annelids
is almost identical, showing a very
close relationship among the three. It
is also similar to molluscan development.
Some authors group the four
phyla into a supraphyletic assemblage
called “Trochozoa” because of the
common possession of a trochophore
larva. Other similarities, too, point to
close relationship of the sipunculans to the echiurans and annelids, such as the
nature of the nervous system and body
wall. Sipunculans and echiurans are
not metameric and thus are more primitive
in that characteristic than annelids.
They probably represent collateral
evolutionary lines that branched from
protoannelid stock before the origin of
Several characters suggest relationship
of Pogonophora to Annelida, as
we noted previously. Molecular sequence
analysis supports close relationships
of Echiura, Sipuncula, and
Pogonophora to Annelida and Mollusca
Phylogenetic affinities of Pentastomida
are uncertain. They have some
similarities to Annelida. Their larval appendages and molting cuticle, however,
are arthropod characteristics.
Their larvae resemble tardigrade larvae.
Most modern taxonomists align
them with arthropods, however, and
evidence is accumulating that they are
most closely related to the crustacean
subclass Branchiura. This evidence
includes similarities in morphology
of their sperm and in base
sequences of ribosomal RNA. If pentastomids
really are close to the
branchiurans, then their status as a
phylum should be revoked, and they
should be classified as crustacean
Onychophorans share a number of
characteristics with annelids: metamerically
arranged nephridia, muscular
body wall, pigment cup ocelli, and ciliated
reproductive ducts. Characteristics shared with arthropods include the
cuticle, tubular heart and hemocoel
with open circulatory system, presence
of tracheae (probably not homologous),
and large size of the brain.
Unique characteristics include oral
papillae, slime glands, body tubercles,
and suppression of external segmentation.
Some authors believe the onychophorans
should be included with
the arthropods, but that would involve
redefining the phylum Arthropoda.
Manton* recommended placing the
Onychophora with the myriapods and
insects in the phylum Uniramia. However,
most authors believe that the differences seem to warrant keeping
them in a separate phylum (Figure
21-17). Sequence analysis supports
placement of Onychophora in Ecdysozoa.
Affinities of tardigrades are among
the most puzzling of all animal groups.
They have some similarities to rotifers,
particularly in their reproduction and
their cryptobiotic tendencies, and some
authors have called them pseudocoelomates.
Their embryogenesis, however,
would seem to put them among the
coelomates. The enterocoelic origin of
the mesoderm is a deuterostome characteristic.
Other authors identify several
important synapomorphies that
suggest grouping them with arthropods
(Figure 21-17). Sequence analysis supports
alignment with arthropods in
Recent discoveries of Cambrian
fossil pentastomids and tardigrades
and additional fossil onychophorans
strongly suggest that these small phyla arose during the Cambrian explosion,
just as did the major phyla. Because
this period was long before terrestrial
vertebrates evolved, the identity of the
hosts for Cambrian pentastomids
remains enigmatic; some authors have
suggested that they might have been
Cladogram depicting hypothetical relationships of Onychophora and Tardigrada to arthropods. Onychophorans
diverged from the arthropod line after
development of such synapomorphies as hemocoel and growth by ecdysis.
They share several primitive characters with annelids, such as metameric
arrangement of nephridia, but molecular
evidence indicates inclusion in Ecdysozoa, not Lophotrochoza. Note that the tracheal system of onychophorans is
not homologous to that of arthropods but represents a convergence.