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.
Hemichordata and Its Key Characteristics
During summer the juvenile phase of this, at first considered a distinct species for it lacks hepatic sacculations, reproduces by cutting off small pieces from the tail end forward. Thus, the larva gradually changes into the adult. Body soft, fragile, vermiform and divisible into proboscis, collar and trunk. By ingesting sand or mud, Balanoglossus feed on diatoms, protozoans, other microbes, and organic waste. The tornaria larva shows a very striking resemblance with the auricularia larva and specially with bipinnaria of Asteroidea. Between the mouth and anus, the alimentary canal can be distinguished into four regions buccal tube, pharynx, oesophagus, and intestine. It is claimed that in some species a few neurosensory cells form photoreceptors sensory to light.
It seems likely that cephalorhynchs are descendents of xenusians, which lost walking appendages and acquired a retractable proboscis as well as a vermifom body during adaptation for a burrowing lifestyle. The glomerulus is made up of several blind tubular projections formed by the peritoneum covering the buccal diverticulum, central sinus and heart vesicle. Balanoglossus is considered a connecting link between chordates and non chordates because it shows characteristics which are present in both the phyla. It is passed out from the anus as castings. The remaining posterior part of the archenteron marks the future gut or alimentary canal. In some genera, the genital ridges are so prominent that they form a pair of wing-like lateral folds, the genital wings, but in other genera folds are absent.
Hemichordata: Phylum Of The Amazing Acorn Worms 
They are worm-like animals with soft bodies mostly elongated. Of the given options, Saccoglossus is a hemichordate. Saccoglossus pygmaeus is the world's smallest Enteropneusta species, measuring 2 to 3 centimetres in length. The protocoel becomes triangular in shape. Genetic control of dorsal-ventral hemichordate patterning: Many of the genetic markers identified in this group are also found in chordates or are homologous to chordates in some way, as much of the genetic work done on hemichordates has been done to compare with chordates. The peribuccal arteries unite together ventrally to form a single longitudinal ventral vessel.
The ventral vessel continues up to the anus and gives off lacunar networks all along the alimentary canal. The details of the branchial apparatus having tongue bars, M-shaped skeletal rods and synapticula are exactly like those of Amphioxus. Differences: But the inclusion of hemichordates in phylum Chordata cannot be justified on the basis of a few similarities which are more than outweighed by important differences. A mid-ventral keel is present on the median plate Fig. General Characters of Hemichordata 2.
Males and females dwell in separate tubes because the sexes are dioecious. Image will be Uploaded Soon Acorn worms are worm-like invertebrates that live alone. The greatest and the most convincing resemblance lies in the method of formation and arrangement of coelomic cavities. Phylum Hemichordata is very important for the study of vertebrates. The cleavage is holoblastic, almost equal and mostly of the radial type. Classification The Enteropneusta, often known as acorn worms, and the Pterobranchia, which includes graptolites, are the two classes of Hemichordata.
Origin of coelom is similar in both Hemichordata and Cephalochordata, it is enterocoelous. The gland cells of pharynx and oesophagus also secrete enzymes. The reproductive organs make their appearance, probably from the mesoderm. An individual lives in a U-shaped burrow excavated by the proboscis. In its burrow Balanoglossus lies in a twisted condition but its anterior and posterior extremities are straight. Respiratory System of Balanoglossus: The respiratory organs of Balanoglossus comprise: 1 The branchial portion of pharynx bearing gill-slits 2 The branchial sacs that open out through gill-pores 1. Chordates are metamerically segmented animals, this segmentation is clearly shown by the muscular, nervous, circulatory, and excretory systems, whereas there is a total absence of segmentation in hemichordates.
Asexual budding produces a metasome, or trunk, which includes a looped digestive system, gonads, and a contractile stalk that binds people to other individuals of the colony. Pterobranchs are minute, sedentary, colonial and tube-building forms. In Balanoglossus, the dorsal tubular nerve cord is confined to the collar region only. Backward movement of food through the alimentary canal is maintained by the cilia lining its walls. The excretory peritoneal cells of glomerulus contain yellow or brown granules, probably of excretory substances. They are colony animals that lived mostly from the Upper Cambrian to the Lower Carboniferous periods.
Affinities of Hemichordata with Chordates, Annelids & Echinoderms
The mouth remains open constantly. Representative Types : 1. As a result, these sea worms are of tremendous importance in the study of chordate development's origins. Sense Organs of Balanoglossus : In Balanoglossus, the sense organs are poorly developed. If you get stuck do let us know in the comments section below and we will get back to you at the earliest.
The body of a Hemichordata is divided into three parts: the proboscis, collar, and trunk. The intestine opens out through the anus situated at the extreme hind end of the body. Tornaria larva during a blackwater dive off the coast of the big island of Hawaii, USA The males also release their sperm into the water. This arrangement exists even today in most books. The skeleton of the proboscis is formed by the nasal capsule.
Why Balanoglossus is called tongue worm? The alimentary canal has the same shape and the same divisions into foregut, stomach and intestine in hemichordate and echinoderm larvae. Cilia covering the proboscis direct the mucous string, containing food particles towards the preoral ciliary organ at the base of the proboscis. According to available evidences maturation starts some four hours before ovulation and that the egg is generally in the metaphase of the first meiotic division when shed. Pterobranchs seem to be filter-feeders that live in a collagenous tubular form called a coenecium. Behind collar is a trunk which contains the digestive and reproductive organs. Is a name of group of Chordata? They have separate sexes.