Hemichordata: Definition, Characteristics, Classification, Meaning, Methods, Topic

Introduction To Hemichordata

Hemichordata is a marine invertebrate phylum belonging to the group of deuterostomes and shows the link between invertebrates and vertebrates. They have three body sections, and include the head called the proboscis, a collar with gill openings and then a trunk where they have the nerve cord. There are solitary and colonial kinds of hemichordates on Earth.

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It is significant due to its position as a link between chordates, the phylum of which vertebrates are a part, and other deuterostomes such as echinoderms, which gives a better understanding of the early evolution of chordates. However, hemichordate is relatively a small group of organisms in terms of species but they are of tremendous importance to the study of the evolution of fabulous marine wildlife.

Taxonomy And Classification Of Hemichordata

The taxonomy is described below-

Overview Of Classification Levels

Hemichordata is a small phylum within the animal kingdom located within the eumetazoan subkingdom. Other animals are ranked under the superphylum Deuterostomia together with the Chordata and Echinodermata. Within Hemichordata, there are three classes: Three main classes, namely Enteropneusta, Pterobranchia, and Planctosphaeroidea with differences in the shape of the organism, habitat and other peculiarities of their existence.

Three Main Classes Of Hemichordates

Enteropneusta

Hemichordates are grouped into three classes, with enteropneusts, or acorn worms, being the most diverse and the best known. It is a benthic group that parasitically occupies the marine sediment and has a proboscis, collar, and trunk. Their locomotion and feeding are performed with a proboscis that can be, extended and withdrawn, allowing them to burrow as well as filter feed.

Pterobranchia

Colonial species of mesohyls called pterobranches are distributed in deep-sea habitats and are of the hemichordate class. They create colonies using structures called stolons and have a feeding organ in the form of a lophophore. Pterobranchs feed by filtering the water picking food particles with the help of cilia.

Planctosphaeroidea

Planctosphaerids as a class are not very popular among the hemichordates and are usually existed in planktonic zones in seas. They are gelatinous and are frequently spheroidal in form and will often have a mucous cuticle and cilia for locomotion. Because of this plan, planctosphaerids are considered to bear a less derived body organization than enteropneusts and pterobranchs.

Morphology And Anatomy Of Hemichordata

The morphology and anatomy are described below-

General Body Structure

Hemichordates have a body structure that is divided into three parts, which include the proboscis, the collar, and the trunk. These animals can be either solitary or belong to the group of colonial marine animals in the taxonomy of the types of animals; they are the transitional link between invertebrates and vertebrates. The two general features present in their anatomy include a ventral nerve cord and pharyngeal gill slit which makes them to be classified under deuterostomes.

Detailed Anatomy Of Enteropneusta

Proboscis

The proboscis is an anterior projection which is utilized for eating and moving in the substrate. It has muscles for locomotion and the genesis of organs and structures for perceiving sensations of the surroundings.

Collar

Found at the posterior to the proboscis, the collar contains the gill slits that are used to feed the animal by filter feeding and for respiration. It links the proboscis with the trunk.

Trunk

It rises backwards from the collar and houses all or most of the body cavity (coelom), the digestive system, and the generative apparatus. Self-fertilization is also seen in it and it may have lateral outgrowths known as mesenteries which contain supporting tissues and arrangement.

Special Features in Pterobranchia and Planctosphaeroidea

Pterobranchia: These hemichordates are colonial with the individuals living in tubes and this characteristic is similar to colonial marine invertebrates. They have elongated arms called “lophophores “ which are used in feeding and respiration as they are stationary animals.

Planctosphaeroidea: Planctosphaeroideans consist of swimming adult exclusively planktonic Hemichordates in possession of a gelatinous body with a chitinous shield. Their body is shaped differently to enable them to move and feed on plankton around the aquatic environment.

Reproduction And Life Cycle

The life cycle is described as-

Reproductive Strategies

Hemichordates use different reproductive means; both sexual and asexual means are recognized among these animals. Sexual reproduction means that there is external fertilization where the eggs and sperm are released into the water and external fertilization occurs. This means that there are differences in the reproductive patterns among the species some are Bisexual and possess both the male and female reproductive systems. Some colonial species of the genus might reproduce asexually using budding or fragmentation, which can increase the population size exponentially.

Life Cycle Of Hemichordates

In many kinds of hydrogenases, there is a complicated morphogenesis that changes larval forms to the forms of adult hemichordates. Fertilization is external and the reproductive process is started when the gametes are discharged into the water. The resulting embryos give rise to free-swimming larvae sometimes with ciliary strips for their movement. Larvae go through a complete transformation and affect the bottom of the ocean or surfaces of objects to develop a life stage that is characteristic of a fully-grown life form.

Developmental Stages

The hemichordate life cycle is rather simple and can be divided into several developmental stages starting from fertilization up to the adult stage. Cleavage occurs after fertilization where the zygote forms a blastula. This stage then develops into a gastrula with three germ layers: ectoderm, mesoderm, and endoderm layers are the three prominent germ layers. The post-larval stage of juveniles differs with each species, but the larval form of many species is called a bilaterally symmetrical tornaria larva. Life transformation from the larvae to the adult is a crucial and complex developmental process; during this process, most of the body structures and organizational patterns of the adult Heterochordata, such as adult proboscis, collar, and trunk, are formed.

Physiology And Adaptations

The physiology and adaptations are described-

Feeding Mechanisms

Based on the species and habitat of the Hemichordates, the species feeds on various types of foods. Some of these animals are classified as suspension feeders and they hang on ciliated organelles or mucus-covered tentacles to feed on plankton and particles floating in the water. Some are deposit feeders where they obtain their food by filter feeding from the bottom of the sediment with the help of proboscis extensions or tentacles. Due to this flexibility in feeding strategies, hemichordates can occupy many different ecological stratifications and locations in the marine ecosystem.

Respiratory System

Laterally, both ends are provided with ventral and dorsal gill openings through which exit water that is breathed in by hemichordates but does not possess gills or lungs. However, respiration takes place through diffusion across the skin and/or through specialized regions of the body wall that possess thin-walled chambers. This diffusion-based respiration is enough for their size and skeletal structure which is comparatively smaller and less complicated as compared to mammals. Some species may also have structural features that concern the skin or changes in beating patterns that can increase oxygen absorption when it is low or high.

Circulatory System

Hemichordates are solely equipped with a basic form of circulatory system comprised of a dorsal and a ventral vessel with the bodily cavity known as the coelom. Contrary to the vertebrates that possess a closed circulatory system, hemichordates make use of the coelom for the distribution of nutrients, gases, and metabolic wastes. This system is consistent with their lower metabolic requirements and it is suitable for the size/physiological needs within the aquatic ecosystem.

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