Traditionally, Panarthropoda (Euarthropoda, Onychophora, Tardigrada)
are regarded as being closely related to Annelida in a taxon Articulata,
but this is not supported by molecular analyses. comparisons of gene
sequences suggest that all molting taxa (Panarthropoda, Nematoda, Nematomorpha,
Priapulida, Kinorhyncha, Loricifera) are related in a monophyletic taxon
Ecdysozoa. An examination of the characters supporting Articulata reveals
that only segmentation with a teloblastic segment formation and the existence
of segmental coelomic cavities with nephridia support the Articulata while
all other characters are modified or reduced in the panarthropod lineage.
Another set of characters is presented that supports the monophyly of Ecdysozoa:
molting under influence of ecdysteroid hormones, loss of locomotory cilia,
trilayered cuticle and the formation of the epicuticle from the tips of
epidermal microvilli. Comparative morphology suggests Gastrotricha as the
sister group of Ecdysozoa with the synapomorphies: triradiate muscular
sucking pharynx and terminal mouth opening. Thus there are morphological
characters which support Articulata but molecular as well as morphological
data advocates Ecdysozoa. Comparison of both hypotheses should prompt further
thorough and targeted investigations.
Figure 1. Concurring hypotheses: Panarthropoda are
either the sister taxon of Annelida within the Articulata (A) or of Cycloneuralia
Figure 2. Resulting cladogram of the analysis of 18S rRNA
gene sequences from Aguinaldo et al. (1997). Protostomia (Gastroneuralia),
Ecdysozoa and Spiralia (bold lines) are strongly supported groups.
The branching patterns within Ecdysozoa and Spiralia are not always well
supported and are therefore not necessarily representative.
Figure 3. Character evolution under the Articulata hypothesis.
Several plesiomorphic (1) or probable synapomorphic (2) characters of the
stem lineage of Articulata are reduced or modified in the panarthropod
stem lineage. Characters of (3) are probable synapomorphies of Annelida
and Panarthropoda. Characters are: 1. Collagenous cuticle, trochophore-like
larva, spiral cleavage. 2. One pair of coelomic cavities per segment,
one pair of metanephridia per segment, ventral nerve cord with one pair
of ganglia per segment, one pair of appendages per segment, one pair of
palps (?). 3. Metameric organization, teloblastic segment formation,
the general existence of coelomic cavities and nephridia. 4. Autoapomorphies
of Panarthropoda: chitinous cuticle, ecdysis. 5. Probable
autapomorphies of Annelida: one pair of palps?, nuchal organs?, parapodia?,
arrangement of setae?
Figure 4. Proposed relationships under the Ecdysozoa hypothesis:
Panarthropoda and Cycloneuralia are sister taxa, the closest sister group
of Ecdysozoa is Gastrotricha. Characters are: 1. Protostomia (Gastroneuralia):
dorsofrontal cerebral ganglion with inner neuropil, longitudinal nerve
cords including one pair of ventrolateral nerves. 2. (Gastrotricha
+ Ecdysozoa): muscular sucking pharynx, terminal mouth opening, bilayered
cuticle including a trilaminate epicuticle. 3. Ecdysozoa: molting
of cuticle by ecdysteroid hormones, loss of locomotory ciliachitinous endocuticle,
secretion of epicuticle by the tips of epidermal microvilli. 4.
Panarthropoda: segmental organization, segmental locomotory appendages.
5. Cycloneuralia: circumpharyngeal brain with an equally thick ring,
brain with antero-posterior sequence of pericarya - neuropil - pericarya.
6. Nematoida: reduction of circular musculature.
7. Scalidophora: introvert with scalids.
Figure 5. Ultrastructural cross section through an early
embryo (coiled stage) of Epiperipatus biolleyi (Onychophora).
coe, embronic coelomic cavity lined by undifferentiated cells with rudimentary
cilia; ect, ectoderm; int, embryonic intestine.
Figure 6. Molting in euarthropods (A), tardigrades (B)
and nematodes (C). A after Foelix (1982); B after Marcus, redrawn
from Ruppert and Barnes (1994); C after Ruppert and Barnes (1994).
Figure 7. A. Molting in Nectonema munidae (Nematomorpha):
the larval cuticle (lc) is replaced by the adult cuticle (ac). ep,
epidermis. B. Trilayered cuticle of the larva of Halicryptus spinulosis
(Priapulida) in the neck region. epi, epicuticle; exo, exocuticle;
endo, endocuticle; ep, epidermis. C: Trilayered cuticle of
Epiperipatus biolleyi (Onychophora). Note the trilaminate
character of the epicuticle. D: Molting in Macrobiotus hufelandi
(Tardigrada). A new epicuticle is formed by secretion of numerous
separate patches (arrows). E: Formation of a new epicuticle
by the tips of epidermal microvilli in a scalid of a larval Halicryptus
spinulosus (Priapulida). Note that the epicuticle is secreted
in separate patches (arrows). C by B. Walz, Potsdam with kind permission
of Springer Verlag, Berlin.
Figure 8. Triradiate pharyngeal lumina in onychophorans and nematodes. In an early (B) and slightly older (A) embryo of Opisthopatus blainvillei (Onychophora) at the level of the brain (A) and further posterior (B). b, brain; pe, pharyngeal epithelium; sag, duct of salivary gland; slg, duct of slime gland; vnc, ventral nerve cords; vo, ventral organ. C. In Enoploides spiculohamatus (Enopida, Nematoda). pg, pharyngeal gland. D: In an undetermined freshwater nematode. C by M. Stohr, Gottingen.