Evolution and systematics
As the echiurans’ physique has no massive onerous elements, fossils of these animals are uncommon. There are two fossils for this group: a fossil from Illinois, United States, dated from the Late Carboniferous and one other fossil from Namibia, dated from the Late Cambrian. A free-swimming trochophore larva is current in echiurans, sipunculans, mollusks, and annelidans, which suggests a phyllogenetic relationship. Echiurans and annelids have many options in frequent. The most necessary difference between them is the absence of segmentation in Echiura. Some authors contemplate echiurans to be correctly placed inside the phylum Annelida, although different research have proven that echiurans and pogonophorans have a detailed affinity and each could also be nearer to mollusks than to annelidans or sipunculans. The phylum Echiura encompasses about 160 species and 40 genera, divided into two courses: Echiuridea, with three orders and 4 households (Echiuridae, Bonellidae, Ikedidae, and Urechidae), and Sactosomatidea, with a single household, Sactosomatidae, with one species, S. vitreum.
Echiurans, often known as spoon worms, have a physique divided in two distinct areas: a sausage-shaped saccular, nonsegmented trunk and a ribbon-like proboscis on the anterior finish. The size of the trunk might vary from 0.39 in (1 cm) up to >19.6 in (>50 cm) and could also be grey, darkish inexperienced, reddish brown, rose, or pink. It could also be thick or skinny, smoothed or roughened by glandular or sensory papillae. Internally, layers of muscle tissue are answerable for peristaltic actions of the trunk. A pair of chitinous golden-brown chaetae often happens ventrally on the anterior a part of the trunk. Some echiurans have one or two rings of chaetae across the anus. The proboscis could also be brief or lengthy, scoopor ribbon-like, and flattened or fleshy and spatulate. It is mostly white, rose, inexperienced, or brown. The distal finish could also be truncate or bifid. It is muscular, mobile, and extremely extensible and contractile. It is in a position to lengthen 10 instances its physique size and might attain 3.2–6.5 ft (1–2 m). The ventral floor of the proboscis is ciliated, which helps within the feeding course of. The mouth is situated ventrally on the base of the proboscis and the anus is on the posterior extremity of the trunk.
Echiurans are primarily marine, however some species reside in brackish waters. The majority of spoon-worms are present in intertidal and shallow waters, however there are additionally species dwelling at depths of 32,800 ft (10,000 m).
Echiurans often reside in a U-shaped burrow with each ends of the burrow open. They are discovered primarily in comfortable benthic substrata resembling sand, mud, or rubble, occupying burrows excavated by themselves or by different animals. Some species reside in rock galleries excavated by boring invertebrates, whereas others reside in empty shells, sand-dollar checks, coral or rock crevices, inside lifeless corals, or beneath stones. In basic, some commensals are current contained in the burrow, together with polychaetes, crabs, mollusks, and fishes. The burrow offers a protected, ventilated dwelling, and stays of food discarded by the spoon-worm could also be eaten by the commensals.
Echiurans are sluggish however not sedentary, and animals with no proboscis can swim. One of a very powerful actions is the peristalsis of the trunk, which permits the animal to transfer slowly over the floor and assemble burrows within the sand or mud. The actions by the peristalsis forces water via the tube, allowing the animal to get hold of a provide of oxygen. In basic, the burrow is saved clear and free from particles and fecal matter.
Feeding ecology and weight-reduction plan
The food of echiurans consists of lifeless natural matter and microorganisms that reside on the substratum. Echiurans could also be detritus feeders. They lengthen the proboscis out of the burrow onto the floor of the sediment. The tip of the ventrally ciliated floor collects particles, and glands produce mucus to adhere to these particles. Movements of the cilia conduct particles and mucus to the mouth. The proboscis is then prolonged in a brand new path, and the process is repeated. A number of species are filter feeders. The innkeeper worm, Urechis caupo, constructs a mucous web placed close to the opening of the burrow. Peristaltic actions of the trunk draw water via the burrow, and particles and small organisms are trapped within the web. Ultimately, the worm eats each the web and the food.
Echiurans reproduce strictly by ####ual means. S*xual dimorphism is pronounced solely within the household Bonellidae, wherein the male is far smaller than the feminine. S*xes are separate, and sperm and eggs are often liberated on the identical time in seawater the place fertilization happens, besides in Bonellidae, wherein people endure inside fertilization. Free-swimming and feeding trochophore larvae develop 22 hours to 4 days following fertilization. The larvae might drift within the plankton for up to three months, and through metamorphosis, it will increase in size. The larva settles on the substratum and begins life as an grownup.
No species of Echiura are listed by the IUCN.
Significance to people
Some species of echiurans are generally used as laboratory animals for physiological, embryological, and biochemical research. The substance bonellin has been studied due to its antibiotic properties.