Hemichordata facts for kids
Quick facts for kids Hemichordata |
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Hemichordata
Bateson, 1885
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Hemichordates are a group of worm-shaped marine animals. They are part of a larger group called deuterostomes. Scientists believe they are closely related to echinoderms (like starfish). These animals have been around for a very long time, since the Cambrian period. An important group of their fossils are called graptolites. Most graptolites died out during the Carboniferous period.
Today, there are two main groups of living hemichordates: the enteropneusts and the pterobranchs. Because hemichordates are the closest living relatives to chordates (animals with a backbone, like us!), they are very interesting to scientists. Studying them helps us understand how chordates first developed.
Contents
Body Structure
The bodies of hemichordates are divided into three main parts. These are the proboscis, the collar, and the trunk. They have an open circulatory system, which means their blood isn't always inside vessels. They also have a complete digestive tract. However, the muscles in their gut are not very strong. So, food mostly moves through their gut using tiny hair-like structures called cilia. These cilia cover the inside surface of their gut.
Some hemichordate species have a hollow neural tube at least when they are young. This tube is like a simple nerve cord. Scientists think this might be an old trait they share with the common ancestor of chordates and other deuterostomes.
Life Cycle
Hemichordates develop in two different ways. Some species develop directly, while others develop indirectly. The enteropneusts group includes both types of development.
In indirect development, the animal has a long pelagic larval stage. This means the larvae live in the open ocean. These larvae feed on tiny organisms called plankton. After a while, they change into an adult worm. Species that develop directly skip this long larval stage. Instead, they grow straight into an adult worm.
Acorn Worms
Acorn worms, also known as Enteropneusta, are a class of hemichordates. They are invertebrates, meaning they don't have a backbone. As mentioned, they are closely related to chordates. There are about 70 known species of acorn worms around the world. A common species studied by scientists is Saccoglossus kowalevskii.
All acorn worms live on the ocean floor. They either eat tiny bits of food from the mud (deposit feeders) or filter food from the water (suspension feeders). Some of these worms can grow very long. One species can reach 2.5 meters (about eight feet) in length! However, most acorn worms are much smaller. One group, Balanoglossus, is also called the tongue worm.
Pterobranchs
Pterobranchia is another group of small, worm-shaped animals. They also belong to the Hemichordata phylum. Pterobranchs live inside tubes that they build themselves. These tubes are attached to the ocean floor.
Pterobranchs get their food by filtering plankton from the water. They use tiny hair-like structures called cilia that are attached to their tentacles. There are about 30 known living species in this group.
Scientists have studied pterobranchs using powerful electron microscopes. These studies suggest that pterobranchs are closely related to the extinct graptolites.
Graptolites
Graptolites are very common fossils found from the Palaeozoic Era. They are colonial animals. This means they lived together in groups. We mostly find their fossils from the Upper Cambrian to the Lower Carboniferous periods. One possible early graptolite, Chaunograptus, has been found from the Middle Cambrian.
The name "graptolite" comes from two Greek words. Graptos means 'written', and lithos means 'rock'. Many graptolite fossils look like hieroglyphs or drawings written on the rock. Long ago, Carl Linnaeus even thought they were just "pictures resembling fossils" rather than real fossils. Today, scientists believe they are very similar to the pterobranchs, and might even be a part of that group.
Graptolite Colonies
Each graptolite colony is called a rhabdosome. A rhabdosome has different numbers of branches, which are called stipes. These branches grow from an initial individual. Each new animal in the colony, called a zooid, lived inside a tube or cup-like structure. These structures are called theca. In some colonies, the thecae come in two sizes. Some scientists think this size difference might be due to sexual dimorphism, meaning males and females had different-sized homes.
The number of branches and how the thecae are arranged are important clues. They help scientists identify different graptolite fossils. Their overall shape has often been compared to a hacksaw blade.
Most of the tree-like graptolites are called dendroid graptolites. They appeared earlier in the fossil record, during the Cambrian period. These animals were benthic, meaning they lived on the sea-floor. They were attached by a root-like base.
Graptolites with fewer branches developed from the dendroid graptolites. This happened at the beginning of the Ordovician period. This second type of graptolite was pelagic. This means they floated freely on the surface of ancient seas. Some might have even been attached to floating seaweed by a thin thread. They were a very successful group. They were the most important animal members of the plankton until they died out in the early Devonian period. The dendroid graptolites, however, survived longer, until the Carboniferous period.
Images for kids
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Schematic of embryonic cleavage and development in P. flava and S. kowalevskii
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Amplexograptus, a graptolite hemichordate, from the Ordovician near Caney Springs, Tennessee.
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Spirograptus from the Silurian period.
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Didymograptus from the Ordovician period.
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Thallograptus sphaericola, a dendroid graptolite, attached to the cystoid Echinosphaerites aurantium; Ordovician of northeastern Estonia.
See also
In Spanish: Hemichordata para niños
