Acorn worm
From Freepedia
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| Image:Acorn worm.PNG a species of the genus Saccoglossus | ||||||||
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Harrimaniidae |
The Acorn worms or Enteropneusta are a hemichordate class of invertebrates. Acorn worms are classified in the phylum Hemichordata, closely related to the chordates. There are about 70 species of acorn worms in the world.
All species are infaunal benthos that either may be deposit feeders or suspension feeders. Some of these worms may grow to be very long; one particular species may reach a length of 2.5 meters (almost eight feet), although most acorn worms are much, much smaller.
One genus, Balanoglossus, is also known as the tongue worm.
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Anatomy
The Acorn worm's body is cylindrical. The body is made up of three main parts: the acorn-shaped proboscis, a short fleshy collar that lies behind it, and the long trunk, which is the rest of the body. The creature's mouth is located at the collar behind the proboscis.
The skin is covered with cilia as well as glands that secrete mucus. Some produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators.
Acorn worms move by cilia movements and body contractions.
Acorn worms breathe by drawing in oxygenated water through their mouth. The water then flows out the animal's gills which is on its trunk. This way, the acorn worm actually breathes with part of its gut.
Similarities with Chordates
Acorn worms are considered more highly specialised and advanced than other similarly shaped worm-like creatures. They have a circulatory system with a heart that also functions as a kidney. Acorn worms have the gill-like structure it uses for breathing, a structure similar to that of primitive fish. Acorn worms are thus sometimes said to be a link between vertebrates and invertebrates.
Lifestyle
Acorn worms are rarely seen by humans because of their lifestyle. They live in U-shaped burrows on the sea-bed, from the shoreline down to the depths of 10,000 ft. (3,050 m). The worms lie there with the proboscis sticking out of one opening in the burrow. Acorn worms are generally slow burrowers.
To obtain food, many acorn worms swallow sand or mud that contains organic matter and microorganisms in the manner of earthworms (this is known as deposit feeding). At low tide, they stick out their rear ends at the surface and excrete coils of processed sediments. This is called the cast.
Another method that some acorn worms use to obtain food is to collect suspended particles of organic matter and microbes from the water. This is known as suspension feeding.
Reproduction
Acorn worms have separate genders that release eggs and sperm into the water for external fertilization. In some, eggs develop into free-swimming larvae that look very similar to echinoderm larvae (this suggests vertebrates and echinoderms are linked phylogenically). The larvae eventually settle down and change into tiny acorn worms on the surface and take on the burrowing lifestyle.
