Representation of various representatives of acorn worms
|Gegenbaur , 1870|
The acorn worms (Enteropneusta), together with the wing gills (Pterobranchia), form the gill lobsters (Hemichordata) within the new mouths (Deuterostomia). Both groups are characterized by a functional tripartite structure that also extends to the body cavity, the coelom . Another common feature of the two otherwise very different groups is the formation of a skeletal rod, which is often viewed as a preliminary stage of the notochord in the chordata. It is doubtful, however, whether this is really a homologous structure.
So far, about 70 species of acorn worms are known, which can reach a body length of a few centimeters up to species over 2 meters. The longest species is the acorn worm Balanoglossus gigas , which occurs on the Brazilian coast and has a body length of up to 2.50 meters. Acorn worms are found on the coasts and in the seas around the world. The 3 cm long Saccoglossus pygmaeus can also be found off the coast of Heligoland .
The acorn worms themselves are characterized by a worm-shaped body. This consists of an elongated abdomen or torso (metasoma), a collar region (mesosoma) and an acorn-shaped front part (prosoma), which serves as a muscular drill in the ground and has given them their German name. Behind the collar are gill pores, an outward sign of the developed among them gills intestine .
The mouth opening of the acorn worms lies in the upper area of the mostly orange collar and is surmounted by the glans. In addition to the external muscles and the anterior coelom space (protocoel), this also consists of a heart-like blood space. The glans is stabilized by a skeletal rod, which is called the glans skeleton. The collar contains the middle coelom space (mesocoel), while in the trunk, next to the last section of the coelom (metacoel), mainly the intestines and the sexual organs lie. The intestine ends at the rear end of the worm , whereby the anus is designed as a respiratory organ (Enteropneusta = "intestinal breather").
Way of life
Almost all types of acorn worms dig tunnels in the ocean floor and move forward in it by wave-like muscle contractions. This movement is supported by the skin surface, which is covered with numerous cilia . The corridors are U-shaped and have an exit for the excrement at the rear end of the animal, which are very typical structures, especially in the tidal flats next to the droppings of the lugworm Arenicola marina . A food funnel is formed at the front end, the animals feed on organic particles contained in the mud. The residential buildings are constantly being rebuilt in order to use new food spaces.
Reproduction and development
Acorn worms are separate sexes, with the males and females already externally recognizable by the color of the sexual organs. In many species, these are located in specially trained genital wings on the body of the animals. The fertilization happens outside of the body. For this purpose, the females of the animals living in the tidal area emit a stream of mucus shortly after the low tide , which contains the eggs, and about 20 minutes later the males add their sperm in the same way.
Typical larvae , known as tornaries, hatch from the eggs . These are barrel-shaped to egg-shaped and must go through an extensive metamorphosis to the adult animal , which is similar to that of the echinoderm.
Four families are distinguished within the acorn worms, the structure and construction of the glans (glans skeleton, glans intestine) and some other characteristics, such as: B. the presence of liver sacs and synapticles play a role. According to phylogenomic data, the monophyly of the families Ptychoderidae and Torquaratoridae against each other is questionable, the second family could be nested in the former.
The relationships can look like this, following this data:
A species of acorn worm newly described in 2012 was named Yoda purpurata by the researcher Nick Holland, who specializes in these animals , because the ear-shaped lips of this deep-sea dweller reminded him of the ears of the Yedi knight Master Yoda .
- Johanna T. Cannon, Kevin M. Kocot, Damien S. Waits, David A. Weese, Billie J. Swalla, Scott R. Santos, Kenneth M. Halanych (2014): Phylogenomic Resolution of the Hemichordate and Echinoderm Clade. Current Biology 24: 2827-2832. doi : 10.1016 / j.cub.2014.10.016
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