Priapulida
Encyclopedia
Priapulida is a phylum of marine worm
s. They are named for their extensible spiny proboscis
, which, in some species, may have a shape like that of a human penis
. They live in the mud, which they eat, in comparatively shallow waters up to 90 metres (295.3 ft).
Together with Echiura
and Sipuncula
, they were once placed in the taxon Gephyrea
, but it is now clear that they belongs to Ecdysozoa
, which mostly include microscopic animals, with the exception of the relatively large arthropod
s, velvet worm
s, and the priapulids. Their nearest relatives are probably Kinorhyncha
and Loricifera
with which they constitute the taxon Scalidophora
. They feed on slow-moving invertebrates, such as polychaete
worms.
Priapulid-like fossils are known at least as far back as the Middle Cambrian
. They were likely major predators of the Cambrian period. However, crown-group priapulids cannot be recognized until the Carboniferous. Only sixteen extant species of priapulid worms are known.
ous cuticle that is moulted as the animal grows.
There is a wide body-cavity, it has no connection with the renal or reproductive organs, so it is not a coelom
; it is probably a blood-space or hemocoel. There are no vascular
or respiratory
systems, but the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment
haemerythrin.
The alimentary canal is straight, consisting of an eversible pharynx
, an intestine, and a short rectum. The pharynx is muscular and lined by teeth. The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it.
The nervous system
comprises a nerve ring around the pharynx and a single ventral cord running the length of the body and including a number of ganglia
. It retains its primitive connection with the ectoderm
, forming part of the body wall. There are no specialized sense
organ
s, but there are sensory nerve endings in the body, especially on the proboscis.
The priapulids are gonochoristic, having two separate sexes (i.e male and female) Their male and female organs are closely associated with the excretory protonephridia. They comprise a pair of branching tufts, each of which opens to the exterior on one side of the anus. The tips of these tufts enclose a flame-cell like those found in flatworm
s and other animals, and these probably function as excretory organs. As the animals mature, diverticula arise on the tubes of these organs, which develop either spermatozoa or ova. These sex cells pass out through the ducts.
s. Some newly hatched larvae lack a proper lorica
. Priapulidae hatching morphology includes these features: light-bulb shape; subdivided into introvert, neck and trunk; lacking a true lorica; without a mouth; without pharyngeal teeth; armed with seven or more scalids; and with four posterior tubuli.
, where their soft-part anatomy is preserved, often in conjunction with their gut contents – allowing a reconstruction of their diets. Trace fossils that are morphologically almost identical to the burrows (Treptichnus pedum
) officially mark the start of the Cambrian
period, suggesting that priapulids, or at least close anatomical relatives, evolved around this time.
Uncertain relationship
Stem-group Priapulida
Phylum Priapulida
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s. They are named for their extensible spiny proboscis
Proboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...
, which, in some species, may have a shape like that of a human penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...
. They live in the mud, which they eat, in comparatively shallow waters up to 90 metres (295.3 ft).
Together with Echiura
Echiura
The Echiura, or spoon worms, are a small group of marine animals. They are often considered to be a group of annelids, although they lack the segmented structure found in other members of that group, and so may also be treated as a separate phylum...
and Sipuncula
Sipuncula
The Sipuncula or Sipunculida is a group containing 144-320 species of bilaterally symmetrical, unsegmented marine worms...
, they were once placed in the taxon Gephyrea
Gephyrea
Gephyrea is a now-dismantled taxon that was considered to be a class of annulated worms, containing the three modern phyla Echiura, Sipuncula, and Priapulida....
, but it is now clear that they belongs to Ecdysozoa
Ecdysozoa
Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...
, which mostly include microscopic animals, with the exception of the relatively large arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s, velvet worm
Velvet worm
The velvet worms are a minor ecdysozoan phylum. These obscurely segmented organisms have tiny eyes, antennae, multiple pairs of legs and slime glands. They have variously been compared to worms with legs, caterpillars and slugs...
s, and the priapulids. Their nearest relatives are probably Kinorhyncha
Kinorhyncha
Kinorhyncha is a phylum of small marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos...
and Loricifera
Loricifera
Loricifera is a phylum of very small to microscopic marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their size ranges from 100 µm to ca....
with which they constitute the taxon Scalidophora
Scalidophora
Scalidophora is a group of marine pseudocoelomate invertebrates, consisting of the three phyla Kinorhyncha, Priapulida, and Loricifera. The members of the group share a number of characteristics, including introvert larvae and moulting of the cuticle...
. They feed on slow-moving invertebrates, such as polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worms.
Priapulid-like fossils are known at least as far back as the Middle Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
. They were likely major predators of the Cambrian period. However, crown-group priapulids cannot be recognized until the Carboniferous. Only sixteen extant species of priapulid worms are known.
Anatomy
Priapulids are cylindrical worm-like animals, ranging from 0.5 to 20 cm (0.196850393700787 to 7.9 ) in length, with a median anterior mouth quite devoid of any armature or tentacles. The body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges. The body is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx. Some species may also have a tail or a pair of caudal appendages. The body has a chitinChitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous cuticle that is moulted as the animal grows.
There is a wide body-cavity, it has no connection with the renal or reproductive organs, so it is not a coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
; it is probably a blood-space or hemocoel. There are no vascular
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
or respiratory
Respiratory system
The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...
systems, but the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment
Respiratory pigment
A respiratory pigment is a molecule, such as hemoglobin in humans, that increases the oxygen-carrying capacity of the blood. The four most common invertebrate respiratory pigments are hemoglobin, haemocyanin, haemerythrin and chlorocruorin...
haemerythrin.
The alimentary canal is straight, consisting of an eversible pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
, an intestine, and a short rectum. The pharynx is muscular and lined by teeth. The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it.
The nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
comprises a nerve ring around the pharynx and a single ventral cord running the length of the body and including a number of ganglia
Ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
. It retains its primitive connection with the ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
, forming part of the body wall. There are no specialized sense
Sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...
organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s, but there are sensory nerve endings in the body, especially on the proboscis.
The priapulids are gonochoristic, having two separate sexes (i.e male and female) Their male and female organs are closely associated with the excretory protonephridia. They comprise a pair of branching tufts, each of which opens to the exterior on one side of the anus. The tips of these tufts enclose a flame-cell like those found in flatworm
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s and other animals, and these probably function as excretory organs. As the animals mature, diverticula arise on the tubes of these organs, which develop either spermatozoa or ova. These sex cells pass out through the ducts.
Reproduction and development
Nothing is known of development, although the eggs hatch and go through development stages that resemble adults, with a vague superficial resemblance to rotiferRotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s. Some newly hatched larvae lack a proper lorica
Lorica
Lorica is a Latin word literally meaning body armour and may refer to:* A number of types of Roman armour:**Lorica hamata, a hauberk of mail**Lorica manica, armguard**Lorica musculata, a folk term for a muscle cuirass...
. Priapulidae hatching morphology includes these features: light-bulb shape; subdivided into introvert, neck and trunk; lacking a true lorica; without a mouth; without pharyngeal teeth; armed with seven or more scalids; and with four posterior tubuli.
Fossil record
Stem-group Priapulids are known from the Middle Cambrian Burgess shaleBurgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
, where their soft-part anatomy is preserved, often in conjunction with their gut contents – allowing a reconstruction of their diets. Trace fossils that are morphologically almost identical to the burrows (Treptichnus pedum
Treptichnus pedum
Treptichnus pedum is regarded as the earliest widespread complex trace fossil...
) officially mark the start of the Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period, suggesting that priapulids, or at least close anatomical relatives, evolved around this time.
Classification
- "Class" Palaeoscolecida
Stem-group Priapulida
- Class †Archaeopriapulida
-
-
- Genus †Ancalagon
- Genus †Anningvermis
- Genus †Corynetis
- Genus †OttoiaOttoiaOttoia is the most abundant priapulid of the Cambrian Burgess Shale formation of British Columbia. It is a stem-group priapulid worm that averaged about 80 mm in length.- Ecology :...
- and more...
-
-
Phylum Priapulida
- Class Priapulimorpha
- Order PriapulimorphidaPriapulimorphidaPriapulimorphida is the sole order within the Priapulimorpha, one of the three extant Priapulid classes....
- Family Priapulidae
- Genus Acanthopriapulus
- Genus Priapulopsis
- Genus Priapulus
- Family TubiluchidaeTubiluchidaeTubiluchidae is one of the two families of Priapulimorphidan priapulid worm....
- Genus Meiopriapulus
- Genus Tubiluchus
- Family Priapulidae
- Order Priapulimorphida
- Class Halicryptomorpha
- Order Halicryptomorphida
- Family Halicryptidae
- Genus HalicryptusHalicryptusHalicryptus is the sole genus of its class of priapulid worms, and grows to great size. It has an important effect on the structure of soft-sediment communities.This genus contains the following species:* Halicryptus higginsi...
- Genus Halicryptus
- Family Halicryptidae
- Order Halicryptomorphida
- Class Seticoronaria
- Order Seticoronarida
- Family Maccabeidae
- Genus Maccabeus
- Family Maccabeidae
- Order Seticoronarida