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Arthropod Oddments |
Class: (various)
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Is it a crab,
a spider or something else? . . . . . . |
A few living arthropods have been discovered and found to have aberrant structural
features which make them difficult to place, with any certainty, into one or other of the four main
arthropod groups - the crustaceans, the myriapods, the arachnids, or the insects. Consequently, these
arthropod 'oddments' are usually classified as separate groups, and generally within several minor
Classes of the Arthropoda - the Onychophora (velvet worms), the Tardigrada (water bears),
the Pentastomida (tongue worms) and the Pycnogonida (sea spiders). In addition to these minor
groups, all of which are represented by living animals, there is also a Class of extinct marine arthropods,
the Trilobita (trilobites), known only from their fossil remains. On this page you can find a brief
account of the various 'odd' animals belonging to these minor and extinct arthropod groups. |
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Onychophora (Velvet Worms) |
The Onychophora, commonly called velvet worms, form a small group of about 70 known
species of caterpillar-like, terrestrial invertebrates confined to the tropical rainforests of Central
and South America, Australia, Africa, Asia and a few other regions. They cannot tolerate dry habitats
and live in moist places among leaf litter, under stones and fallen logs, or in crevices and galleries
in the soil. They avoid light and usually remain well concealed from casual observation. All the velvet
worms are predatory, feeding on small invertebrates such as crickets, spiders, and woodlice. They catch
and subdue their prey by spitting a quick-hardening slime from projections (called oral papillae) near
the mouth. They also squirt slime in self-defence when attacked or disturbed.
Onychophorans range in size from about 2-15 cm in length and the body is covered with
a thin, dry cuticle that is finely ridged and convoluted, giving a velvety appearance to the surface,
hence the common name of the group. Body colouring varies from dark bluish-grey to reddish-brown, usually
with a darker median stripe along the back. The head appendages include a pair of antennae and jaws, and
the elongate body consists of numerous trunk segments (around 15-45 depending on the species), each bearing
a pair of short, thick legs. The legs differ from true arthropod legs by lacking joints, but they do have
terminal claws which superficially resemble the leg-claws of true arthropods. The best known velvet worms
belong to the genus Peripatus. |

Velvet Worm (Peripatoides)
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The anatomy of onychophorans is a mixture of annelid-like and arthropod-like features,
and in evolutionary development they are generally considered intermediate between the annelid worms and
the arthropods - perhaps descendants of a line that branched off close to the primitive annelid-arthropod
stock. There is certainly no doubt that the Onychophora are very ancient, since their fossil remains date
back some 520 million years to the Cambrian*.
Photo:
Encyclop�dia Britannica 2004 Ultimate Reference Suite DVD
Copyright © 1994-2003 Encyclop�dia Britannica, Inc. |
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Tardigrada (Water Bears) |
The Tardigrada, or water bears, are a cosmopolitan group of free-living invertebrates,
with about 350 known species. All are minute, more or less translucent animals, generally 1 mm or less
in length, and live in various habitats, including damp moss, on flowering plants, in sand, in freshwater,
and in the sea. They have a well developed head region and a short body composed of four fused segments.
Each body segment has a pair of short, stout, unjointed legs, usually terminated by several sharp claws
(in one exceptional genus the legs are two-jointed). Most of the plant-eating tardigrades feed by piercing
individual plant cells with their stylets (spear-like structures near the mouth) and then sucking out the
cell contents. A few species are predacious carnivores.
A remarkable feature of the tardigrades is their ability to withstand extremely low
temperatures and desiccation. Under unfavourable conditions, they go into a resistant state of suspended
animation. Laboratory studies have shown that specimens kept for eight days in a vacuum and then exposed
for several hours to a temperature of -272oC (-458oF) came to life again when they
were brought to normal room temperature. Sixty percent of specimens kept for 21 months in liquid-air at
a temperature of -190oC (-310oF) also revived. In nature, tardigrades are often
dispersed by wind and water while in such resistant states, and by this means they can quickly colonize
new habitats. |
 (top)
 (side)
Water Bear (Echiniscus)
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The evolutionary position of the Tardigrada is generally thought to lie somewhere
between annelid worms and arthropods, and like the Onychophora described above, they may have survived
relatively unchanged from very ancient origins at or near the base of the whole arthropod ancestral
tree. |
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Pentastomida (Tongue Worms) |
The Pentastomida, or tongue worms, are a small group of about 70 species of worm-like
or fluke-like endoparasites of back-boned animals (vertebrates). They are mostly found in the tropics and
subtropics, although those with warm-blooded hosts may also occur in cold or temperate regions. Adult tongue
worms are usually whitish in colour and range in size from a few millimetres to about 15 cm in length. The
front part of the body (termed the cephalothorax) bears the mouthparts and two pairs of hooks which serve
as organs of attachment to the host animal. The rest of the body (termed the abdomen) is either cylindrical
in form (as in the Porocephalus species illustrated) or broad and flattened (e.g., Linguatula
species), generally with conspicuous segmental rings. The young stages of pentastomids (referred to as
larvae) have a rounded body with stylet-like, piercing mouth-parts at the front-end and two pairs of
clawed legs further back. Apart from fewer legs, they more or less resemble tardigrades in general body
form (see above). The larvae moult several times and eventually loose their limbs as they develop to the
adult stage. |

Tongue Worm (Porocephalus)
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Adult pentastomids live in the respiratory systems (nasal cavities, lungs, etc.) of
carnivorous reptiles, birds and mammals. The flesh-eating animals, which harbour the adult parasites,
are called primary hosts. This is because tongue worms have a complex life-cycle involving an intermediate
or secondary host, usually a herbivore or omnivorous animal, in which the young stages develop. Eggs are
discharged from the primary host, usually through sneezing or coughing. If they fall on grass or other
vegetation and then become ingested by a suitable plant-feeding secondary host, the eggs hatch into larvae
which burrow through the gut-wall into the neighbouring viscera or body organs of this host, where they
come to rest and form dormant cysts. If an infested secondary host is then eaten by a carnivore (the
primary host), the encysted larvae escape and migrate to the nasal chambers or lungs of this host, where
they moult into adults which attach and anchor themselves by their hooks and mouth-parts to the surface
tissues and begin feeding on the blood of the primary host animal. Pentastomid species belonging to the
genus Porocephalus are mainly parasitic in snakes and rodents, whilst members of the other main
genus of the group, Linguatula, usually parasitize various mammals, including dogs. A few species
are of medical interest because they may infest humans.
The anatomy of adult pentastomids has been so much modified and altered by parasitic
life that their origins and affinities to other invertebrates are rather obscure. However, their larval
stages, with clawed limbs, are reminiscent of the Tardigrada and, like this group, they are generally
thought to lie somewhere between annelids and arthropods in evolutionary development. Some authorities
regard the group as an ancient and specialised off-shoot of the early arachnids. |
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Pycnogonida (Sea Spiders) |
The Pycnogonida, commonly known as sea spiders, are spider-like marine invertebrates
that live on the sea-bed. Their body size ranges from a few millimetres in some shallow-water species
up to 50 cm in length among deep-water species. Most pycnogonids have four pairs of exceptionally long,
thin legs, often six or seven times longer than the body, although in some forms (as in the species of
Pycnogonum illustrated) the legs are relatively short and stout. The legs have terminal claws
and are attached to a four-segmented trunk or cephalothorax, which forms the main part of the body,
with the abdomen behind reduced to a single segment. The mouth lies at the end of an elaborate suctorial
appendage (or proboscis) which is often longer and larger than all the rest of the body. Some species
also have palp-like appendages attached to the cephalothorax on either side of the proboscis. The first
segment of the cephalothorax usually bears four simple eyes, although these are often lacking in the
deep-water species. The cuticle of the body and limbs is very hard and strong, generally brownish in
colour, and often raised into pronounced bumps and tubercles, particularly along the back. |

Sea Spider (Pycnogonum)
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Sea spiders are carnivores and either suck the juices from soft-bodied invertebrates
or browse on hydroids and bryozoans (sedentary corals and coral-like animals). Their reproduction is of
interest since it involves parental care by the males. Until the hatching of the young, the male carries
the egg-mass produced by the female under his cephalothorax, attached to a special pair of appendages,
sometimes called the ovigerous legs. The larval or juvenile stages of many species are parasitic in
hydroids or molluscs. The long legs of pycnogonids are often injured or lost, but evidently they can
repair and regenerate damaged limbs.
There are more than 600 described living species of Picnogonida, and at least one
fossil species from the Jurassic Period* (about 150-200 million years old). The group has sometimes
been affiliated to the arachnids, but the similarities here are fairly tenuous and most modern
classifications place the pycnogonids as a distinct Class of the arthropods on the assumption that
they probably arose from ancient arthropod stock quite independently, either before or close to the
arachnid branch. Several older works note the broad resemblance between juvenile pycnogonids and the
nauplius (or planktonic) larvae of some marine crustaceans. Detailed examination of these larval forms,
however, indicates that their outward resemblance has little or no phylogenetic significance, other
than perhaps lending support to an early, pre-arachnid origin. |
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Trilobita (Trilobites) |
The Trilobita (trilobites) are a group of extinct fossil arthropods, easily recognized
by their distinctive three-lobed body shape - a domed central area running length-wise forming one 'lobe',
and the areas on either side forming the other two 'lobes' - from which the group gets its name. From
front to rear, the body was clearly divided into three regions - the head (or cephalon), followed by the
thorax and then the abdomen (or pygidium) - see illustrations. The head was covered by a carapace (or
cephalic shield), more or less semicircular in shape, with a pair of compound eyes present in most species.
In some forms the rear corners of the cephalic shield were produced backwards to form long spines. The
thorax consisted of several freely articulating segments (up to about 25 in some species), which evidently
allowed the animals to curl up, somewhat after the manner of a present-day woodlouse.
The abominal segments were similar to those of the thorax, but apparently fused together to form a ridged
'tail'.
Although fossil trilobites are often found in abundance and in an excellent state of
preservation, it is only in rare examples that anything can be seen of the lower side and appendages.
These few specimens indicate that paired appendages, essentially similar to those of most cructaceans,
were present on each body segment. The head segments bearing a pair of long, jointed antennae and four
pairs of shorter palp-like sensory and feeding appendages associated with the mouthparts. The thoracic
and abdominal segments, apart from the very last anal segment, each bearing a pair of jointed legs.
Trilobites had a hard external covering (or exoskeleton), probably composed of
chitinous and perhaps calcareous material, like that of modern crustaceans and other arthropods.
Likewise, the exoskeleton was shed periodically to allow growth - shed trilobite 'skins', or portions
of them, are often found as fossils. Most triolobites were relatively small, up to a few centimetres
long, but some grew to large size. A species of Paradoxides, which has been found near Boston
(USA) in rocks of the Middle Cambrian* (about 520 million years old), grew to more than 45 cm in length
and may have weighed as much as 4.5 kg.
Trilobites were exclusively marine animals, but with only fossil remains nothing is
really known about their habits and life-style. Some forms may have been active predators, whereas others
were probably scavengers, and a few may have been plankton-feeders. Fossil evidence indicates that they
probably layed eggs which hatched into a larval stage, similar in appearance to the adult but with fewer
body segments, these increasing in number at each moult as the animal grew and matured. |

Fossil Trilobite

Body Regions of a Trilobite (Calymene)
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Trilobites have been found in the oldest known fossil deposits of the Lower Cambrian*
(about 540 million years old), where they are represented by numerous species. It is evident from this
variety of Cambrian forms that the group, even at that remote period, must already have been very old
and of Pre-Cambrian origin. The group were clearly an important part of the Cambrian fauna, and came to
dominate the seas during the Ordovician Period* (around 440-500 million years ago). In succeeding
geological periods they became less abundant, with a marked decline during the Devonian* (360-410 million
years ago) and eventual extinction of the entire group by the end of the Permian* (around 250 million
years ago).
Nothing is yet known about the Pre-Cambrian ancestors of the trilobites, and hence
there is no direct evidence of the likely origin of the group. A fossil organism called
Spriggina,
which is known from Pre-Cambrian shallow-water marine deposits in Australia, may be ancestral to the
trilobites, as well as to other arthropods, but this is by no means certain. The trilobites show some
resemblance to certain groups of living crustaceans, suggesting some ancestral connection, but in many
respects their anatomical features appear to be more primitive than any other known living or extinct
arthropods, at least amongst those belonging to the major Classes - the Crustacea, Myriapoda, Arachnida
and Insecta. In this respect, a trilobite-like animal is perhaps the closest we can come to picturing
the ancestral form that gave rise to all the modern arthropods, and of course the trilobites themselves.
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(classification of arthropods) |
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