Acorn worm

Acorn worm

Taxobox



name = Enteropneusta
regnum = Animalia
phylum = Hemichordata
classis = Enteropneusta
subdivision_ranks = Families
subdivision =
Harrimaniidae
Protoglossidae

Ptychoderidae

Saxipendiidae

Spengelidae

Torquaratoridae

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 worm in the world, the main species for research being "Saccoglossus kowaleski".

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.

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. One theory is that this three-part body originates from an early common ancestor of all the deuterostomes, and maybe even from a common bilateral ancestor of both the deuterostomes and protostomes.

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 are on its trunk. Thus, the acorn worm breathes about the same way as fish.

imilarities to 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 gill-like structures that they use for breathing, similar to the gills of primitive fish. Hence, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates. Some also have a postanal tail which sometimes shows weak signs of segmentation. An interesting trait is that its three-section body plan is no longer present in the vertebrates, except for the anatomy of the frontal neural tube, later developed into a brain which is divided into three main parts. This means some of the original anatomy of the early chordate ancestors is still present even if it is not always visible.

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 a depth 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 (casts).

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 closely linked phylogenically). The larvae eventually settle down and change into tiny acorn worms on the surface and take on the burrowing lifestyle. Others don't have a larval stage, but develops directly into small juveniles.


Wikimedia Foundation. 2010.

Игры ⚽ Нужен реферат?

Look at other dictionaries:

  • acorn worm — any of several burrowing, often brilliantly colored hemichordates of the class Enteropneusta, usually found in intertidal sand and mud, having an acorn shaped proboscis and collar. Also, acornworm. [1885 90] * * * ▪ hemichordate also called… …   Universalium

  • acorn worm — a′corn worm n. ivt any wormlike marine animal of the phylum Hemichordata, having gill slits and an acorn shaped proboscis and collar …   From formal English to slang

  • acorn worm — noun Date: circa 1889 any of a class (Enteropneusta) of burrowing wormlike marine animals having an acorn shaped proboscis and classified with the hemichordates …   New Collegiate Dictionary

  • acorn worm — noun a burrowing worm like marine animal with a proboscis, a collar, and a long trunk with gill slits. [Class Enteropneusta.] …   English new terms dictionary

  • acorn worm — noun Etymology: so called from the shape of the front part of the body : a worm of the group Enteropneusta …   Useful english dictionary

  • Worm cast — A worm cast or vermicast [http://www.kitsapezearth.com/fact.html KITSAP E Z Earth] ] is a structure created by worms, typically on soils such as those on beaches, [http://www.open2.net/coast/postcard beachlife.html BBC Beach Life] ]… …   Wikipedia

  • acorn — n. the fruit of the oak, with a smooth nut in a rough cuplike base. Phrases and idioms: acorn barnacle a multivalve marine cirriped, Balanus balanoides, living on rocks. acorn worm any marine wormlike animal of the phylum Hemichordata, having a… …   Useful english dictionary

  • Worm — A worm is a common name given to a diverse group of invertebrate animals that have a long, soft body and no legs. There are hundreds of thousands of species of worms, 2,700 of these are earthworms. Animals which are commonly called worms include… …   Wikipedia

  • acornworm — acorn worm n. Any of a class (Enteropneusta) of hemichordate, wormlike animals that inhabit shallow burrows in mud or sand flats of intertidal zones and are equipped with an acornlike proboscis used for digging and collecting food. * * * …   Universalium

  • hemichordate — /hem i kawr dayt/, Zool. adj. 1. belonging or pertaining to the chordates of the phylum Hemichordata, comprising small, widely distributed, marine animals, as the acorn worms. n. 2. a hemichordate animal, having a vertebratelike hollow nerve cord …   Universalium

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”