| Acorn worms | |
|---|---|
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| ByJohann Wilhelm Spengel [de], 1893 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Hemichordata |
| Class: | Enteropneusta Gegenbaur, 1870 |
| Families | |
Theacorn worms orEnteropneusta are ahemichordate class ofinvertebrates.[2] The closest non-hemichordate relatives of the Enteropneusta are theechinoderms.[3] There are 111 known species of acorn worm in the world,[4] the main species for research beingSaccoglossus kowalevskii. Two families—Harrimaniidae andPtychoderidae—separated at least 370 million years ago.[5]
Until recently, it was thought that all specieslived in the sediment on theseabed, subsisting asdeposit feeders orsuspension feeders. However, the early 21st century has seen the description of a new family, theTorquaratoridae, evidently limited to the deep sea, in which most of the species crawl on the surface of the ocean bottom and alternatively rise into the water column, evidently to drift to new foraging sites.[6][7][8][9][10] It is assumed that the ancestors of acorn worms used to live in tubes like their relativesPterobranchia, but that they eventually started to live a safer and more sheltered existence in sediment burrows instead.[11] The body length normally range from 2 centimetres (0.79 in) to 2.5 metres (8 ft 2 in) (Balanoglossus gigas),[12] but one species,Meioglossus psammophilus, only reach 0.6 millimetres (0.024 in).[13] Due to secretions containing elements like iodine, the animals have aniodoform-like smell.[14]
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Most acorn worms range from 9 to 45 centimetres (3.5 to 17.7 in) in length, with the largest species,Balanoglossus gigas, reaching 1.5 metres (5 ft) or more. The body is made up of three main parts: an acorn-shaped proboscis, a short fleshy collar that lies behind it, and a long, worm-like trunk. The creature'smouth is located at the collar behind the proboscis.[16]
The skin is covered withcilia as well as glands that secretemucus. Some produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators. Acorn worms move only sluggishly, using ciliary action andperistalsis of the proboscis.[16]
Many acorn worms aredetritus feeders, eating sand or mud and extracting organic detritus. Others feed on organic material suspended in the water, which they can draw into the mouth using the cilia on the gill bars.[17] Research indicates that the rate of feeding of acorn worms that are detritus feeders is dependent on food availability and flow rate.[18] A groove lined with cilia lies just in front of the mouth and directs suspended food into the mouth and may allow the animal to taste.[16]
The mouth cavity is tubular, with a narrowdiverticulum orstomochord extending up into the proboscis. This diverticulum was once thought to behomologous with thenotochord of chordates, hence the name "hemichordate" for the phylum. The mouth opens posteriorly into apharynx with a row of gill slits along either side. The remainder of the digestive system consists of anoesophagus andintestine; there is no stomach.[16]
In some families there are openings in the dorsal surface of the oesophagus connecting to the external surface, through which water from the food can be squeezed, helping to concentrate it. Digestion occurs in the intestine, with food material being pulled through by cilia, rather than by muscular action.[16]
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.
Acorn worms have anopen circulatory system, in which the blood flows through the tissuessinuses. A dorsal blood vessel in themesentery above the gut delivers blood to a sinus in the proboscis that contains a muscular sac acting as aheart. Unlike the hearts of most other animals, however, this structure is a closed fluid-filled vesicle whose interior does not connect directly to the blood system. Nonetheless, it does regularly pulsate, helping to push blood through the surrounding sinuses.[16]
From the central sinus in the collar, blood flows to a complex series of sinuses andperitoneal folds in the proboscis. This set of structures is referred to as aglomerulus and may have an excretory function, since acorn worms otherwise have no defined excretory system. From the proboscis, blood flows into a single blood vessel running underneath the digestive tract, from which smaller sinuses supply blood to the trunk, and back into the dorsal vessel.[16]
The blood of acorn worms is colourless and acellular.[16]
Acorn worms continually form new gill slits as they grow in size, with some older individuals of species likeBalanoglossus aurantiacus having more than a hundred on each side. The microscopic speciesMeioglossus psammophilus has just a single gill slit. The gills in some acorn worms have cartilaginous support structures.[19] Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft and to the exterior through a dorso-lateral pore (see diagram below). Cilia push water through the slits, maintaining a constant flow. The tissues surrounding the slits are well supplied with blood sinuses.[16]
Aplexus of nerves lies underneath the skin, and is concentrated into both dorsal and ventral nerve cords. While the ventral cord runs only as far as the collar, the dorsal cord reaches into the proboscis, and is partially separated from the epidermis in that region. This part of the dorsal nerve cord is often hollow, and may well be homologous with the brain of vertebrates. In acorn worms, it seems to be primarily involved with coordinating muscular action of the body during burrowing and crawling.[16]
Acorn worms have no eyes, ears or other special sense organs, except for the ciliary organ in front of the mouth, which appears to be involved in filter feeding and perhapstaste.[20] There are, however, numerous nerve endings throughout the skin.[16]
Acorn worms have a Y-shaped nuchal skeleton that starts their proboscis and collar on their ventral side. The length of the horns of the nuchal skeleton varies between species.[21]
Acorn worms have a circulatory system with a heart that also functions as a kidney.[citation needed] Acorn worms have gill-like structures that they use for breathing, similar to the gills of primitive fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates andvertebrates. Some also have a postanal tail which may be homologous to the post-anal tail of vertebrates. 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.
One theory is that the three-part body originates from an early common ancestor of all thedeuterostomes, and maybe even from a common bilateral ancestor of both the deuterostomes andprotostomes.[citation needed] Studies have shown that the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system,[22] they are controlling the development of the different body regions.[23]
The internal relationships within the Enteropneusta are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources.[24][25]
| Hemichordata | |
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 obtainfood, 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.[17]
Acorn worms aredioecious, having separate biological sexes, although at least some species are also capable ofasexual reproduction in the form offragmentation.[26] They have pairedgonads, which lie close to the pharynx and release thegametes through a small pore near to the gill slits. The female lays a large number of eggs embedded in a gelatinous mass of mucus, which are thenexternally fertilized by the male before water currents break up the mass and disperse the individual eggs.[16]
In most species, the eggs hatch intoplanktonic larvae with elongated bodies covered in cilia. In some species, these develop directly into adults, but in others, there is a free-swimming intermediate stage referred to as atornaria larva. These are very similar in appearance to thebipinnaria larvae ofstarfishes, with convoluted bands of cilia running around the body. Since the embryonic development of theblastula within the egg is also very similar to that ofechinoderms, this suggests a closephylogenetic link between the two groups.[16]
After a number of days or weeks, a groove begins to form around the larval midsection, with the anterior portion eventually destined to become the proboscis, while the remainder forms the collar and trunk. The larvae eventually settle down and change into tiny adults to take on the burrowing lifestyle. A few species, such asSaccoglossus kowalevskii, lack even the planktonic larval stage, hatching directly as miniature adults.[16]
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Willey, Arthur (1911). "Balanoglossus". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 3 (11th ed.). Cambridge University Press. pp. 237–239.
