Hemichordata is a phylum of marine invertebrates that includes the acorn worms and the pterobranchs. The acorn worms, also known as enteropneusts, are worm-like animals with a simple, tubular body and a simple digestive system. They are named after the acorn-shaped proboscis, or feeding organ, that protrudes from the front of their body. The pterobranchs, on the other hand, are small, colonial animals that live in tubes and have a more complex anatomy.
One well-known member of the hemichordata phylum is the acorn worm Balanoglossus. Balanoglossus is a burrowing animal that lives in soft sediments at the bottom of the ocean. It has a long, slender body with a series of gill slits along the sides and a simple digestive system. The front part of its body is modified into a proboscis, which it uses to filter small particles from the water for food.
Balanoglossus is an important organism in the study of evolutionary biology because it is thought to be a transitional form between invertebrates and vertebrates. It has many features that are found in both groups, such as a notochord (a flexible rod that provides support and movement), a hollow nerve cord, and pharyngeal gill slits (which are used for respiration and feeding). These features suggest that Balanoglossus may have evolved from a common ancestor with the vertebrates.
Despite its importance in evolutionary studies, little is known about the biology and ecology of Balanoglossus. It is found in shallow, tropical waters, but very little is known about its behavior or reproductive habits. Further research on this fascinating animal could help us understand the early evolution of vertebrates and the relationships between different groups of animals.
Digestive System of Balanoglossus (Hemichordata)
Balanoglossus is a deuterostome, which is similar to Ascidian or marine squirts in that it has branch openings or gill slits. The details of the branchial apparatus having tongue bars, M-shaped skeletal rods and synapticula are exactly like those of Amphioxus. But they have only one character of Chordata, i. They are the typical Acorn Worms. The close affinities of Hemichordata, Echinodermata and Chordata, etc.
ADW: Balanoglossus: CLASSIFICATION
Ans: The two common characteristics between chordates and hemichordates are the presence of tubular nerve cords and pharyngeal slits. The embryo has reached the 16-cell stage once the fourth division has happened. The trunk coelom is separated from the collar coelom by a collar-trunk septum. At the same time, the Palaeoscolecida share a number of char- acters with the lobopod-bearing Cambrian ecdysozoans, the Xenusia. The sperm swim and seek out the eggs, which it is believed they detect chemically. The body elongates and is distinguished into proboscis, collar and trunk by the appearance of two constrictions, and the trunk region is elongated. The chief link between the hemichordates and chordates lies in the pharynx and its gill-clefts.
Balanoglossus: Habitat, Development and Affinities
Phylum Hemichordata involves marine deuterostome animals and is related to chordates. There are certain resemblances between the nervous system of hemichordates and chordates, such as its position, and formation of the dorsal nerve cord from the dorsal epidermis, and the collar cord which often has a neuropore and is comparable with the brain of vertebrates. The four vegetal blastomeres split evenly but unequally, resulting in four large macromeres and four smaller micromeres. The zooids of the genus Rhabdopleura are constantly linked to the remainder of the colony through a stolon system. Filter feeders have mucous secreting glands and cilia on their proboscis. Hemichordates are usually vermiform, solitary, or colonial enterocoelous coelomate animals with an intra-epidermal nervous system and a pre-oral with or without gill-slits and without typical nephridia. The coelomic cavities of collar are completely cut off from the proboscis cavity.
