Different Types Of Phylum Hemichordates And Their Characteristics

Subdivisions and taxonomic rankings are essential in biology to most efficiently group together animal types that may have similar characteristics and share a similar purpose within the animal ecosystem.

There are currently around 35 known animal phyla classes, 12 plant phyla and 7 fungi phyla, however this number changes all the time as new evidence and techniques such as genetic testing are discovered to more accurately group types of animals and plants.

Different Types of Phylum Hemichordates and Their Characteristics

Over the years much research has been put into most known phyla types and this includes Phylum Hemichordata which is made up of a wide range of marine deuterostome animals with very particular appearances and characteristics.

Below we have taken a deep dive into some of the most prominent animals within the phylum hemichordata class along with their particular characteristics and noticeable behavior.

What Is The Phylum Hemichordata?

The Phylum Hemichordata is classed as a subphylum of the phylum chordata however it is also known to be closely related to the Phylum Echinodermata which is considered its sister group.

The group is composed primarily of wormlike marine invertebrates with the actual number of animals being updated as new evidence is found with the total number being around 130 members.

This number is actually quite small compared to other phylum groups, however their importance within research and data analysis should not be understated.

These species have been incredibly important within the study of vertebrate evolution, the fossil record of one group of hemichordates specifically, the graptolites, is very popular and used to better correlate rocks.

All species classed as hemichordates are considered marine deuterostomes and mostly all are recognisable by their threefold division of the body being seen with a pre oral lobe, collar and trunk.

While they can vary in size, these grouped animals are typically found in oceans and on the seafloor and feed on small organic particles.

There are three primary classes of Phylum Hemichordata that are similar in function and purpose, but have slightly different characteristics and appearances.

Enteropneusta

The most famous and recognisable of the three classes, Enteroponeusta are also called ‘acorn worms’ and make up about 70 species within the entire Hemichordata subset and are described as solitary, wormlike bilaterally symmetrical animals.

These worm-like creatures are very distinguishable because of their appearance, specifically the triple division of the body.

The front end of these animals consists of a muscular proboscis and a collar that is used to help burrow into the soft sand sea or mud, and is what gives them their ‘Acorn’ nickname because of the shape.

These creatures vary in length ranging anywhere from about 2 inches all the way up to 6 in some species such as in certain Saccoglossus.

They are also quite unique in the sense that these creatures can have as many as 200 branchial openings through their body.

Essentially all acorn worms reside along the seashore and in water often around 10,000 feet deep.

Many gain their required nutrients by filtering food from seawater that passes into the mouth and then out through the gills in the pharynx.

Additionally, they are very slow burrowers and will use the proboscis to burrow through sediment where they can either deposit food and consume sediment and digest organic matter directly, or suspension feed which refers to collecting suspended particles in the water.

Acorn worms are known to have separate sexes. Their paired gonads are right next to their gills lying in the ‘Trunk’ which is located just behind the collar.

Females of some species will lay a few large eggs which will then hatch either into miniature acorn worms, or into swimming juvenile torn aria larvae who eventually grow into young worms.

Enteropneusts are known to dwell in intertidal zones from the White Sea and Greenland to the South of New Zealand.

Pterobranchia

This class is a bit more obscure, only taking up around 20 of all known species within the Hemichordata group. Pterobranchia act in quite a different way to those species classed as enteropneusta.

Rather than living as individuals, these species act in colonies or pseudo-colonies of minute ‘zooids’.

Within these colonies, the individuals are interconnected by stems, also known as stolons, with the individuals or ‘zooids’ themselves being only about 0.39 inches long and so are incredibly small.

They are also recognisable by their distinct appearance of an elongated body again separated into the proboscis, collar and trunk regions.

They were originally fairly understudied, however they have received much more attention in recent years with new species being discovered in this group as research continues.

While their appearance follows the same template, noticeable differences include the proboscis not necessarily being elongated like the acorn worm but instead having a shield shape to it.

The second division of the body is also made up of branched tentacles working to collect small food particles out of the water.

One major feature of species classed as pterobranchia and what truly differentiates them is how they create and live through a network of tubes known as the coenecium.

This is the case for one of the three genera of pterobranchia, specifically the Rhabdopleura and Cephalodiscus who live in these secrete tubes.

These tubes are made up of a protein called collagen and is secreted by special glands located in the proboscis.

The third genus, Atubaria, in contrast lives on hydroids.

This genus is quite intriguing as a species because while they have been recognised as a potential non-colonial species, information on their actual reproduction and lifestyle is lacking leaving a lot of mysteries to be discovered.

Pterobranchs are considered very rare and can reproduce asexually through the process of budding, or through regular sexual reproduction through the fetilizastion of eggs to produce an embryo.

Graptolithina

The most recently discovered and third class of hemichordates, Graptolithina or grapholites as they are also commonly called have quite a fascinating history.

Graptolites have actually been known for quite a few decades and are common fossils within Ordovician and Silurian rocks.

Different Types of Phylum Hemichordates: Graptolithina

However, because of their unclear appearance being seen as nothing more than tiny sawblades, no one knew how to identify them.

While pterobranchs are known to typically live in encrusted rocks and shells, graptolites since they have been granted more research are thought to have been planktonic as they slowly float or sink throughout the water with it being suggested that some even used gas filled sacs to keep them buoyant.

Because their characteristics are considered similar in a few ways to their pterobranchia cousins, they are either classed as a subset of this class or as an entire class of its own made up of several extinct species of graptolites.

These creatures are thought to have lived between the Cambrian and Carboniferous periods, around 520 to 350 million years ago.

They are shown to have been colonial animals similar to pterobranchs, and they even use similar interconnected tubes to live in.

This is made up of the initial ‘embryonic’ cone-like tube known as the sicula which is then followed by subsequent tubes that are arranged in branches to make up the entire colony known as the rhabdosome.

Graptolites did live on the sea bed and would attach themselves to boulders or cones rooted in the soft mud, this way of living is still used by the very few living graptolites today.

While some graptolite colonies remained small, others are understood to have evolved into massive harvesting arrays living upwards of 20 years.

While some details of graptolites are yet to be fully understood, what has been documented is their struggles being a free floating and sinking species that required them to evolve, giving them the spiral shape appearance that seems to be an adaptation to slow sinking.

Habitat

All three classes of hemichordata can act differently in terms of how they eat, details of their appearance and whether they are independent or colony based creatures, however there are a few similarities that are shared between all of them including their habitat.

All hemichordates are found in oceans all across the world and are marine organisms, the major difference is just where each class situates itself around the water.

Enteropneusta for example tend to live directly along the sea shore and in shallow areas where they can be found in their burrows, however they are also known to live under shells and stones deep in the ocean.

Pterobranchs too live in the depths of the ocean but are situated in their cornelia tubular structures where the colony can feed and stay connected.

Hemichordates are excessively marine animals with the highest reported numbers being located in the North Pacific and North Atlantic Oceans.

All types of hemichordata species are also known to be either deposit or suspension feeders.

Suspension feeding involves capturing food particles suspended in the water and ingesting them, some examples of these particles include zooplankton and detritus.

Deposit feeders in contrast are certain species who will naturally ingest particles that are along the surface of soft sediments.

Appearance And Anatomy

While the exact details can differ slightly between each class and species, one of the recognisable parts of any hemichordata creature is their unique appearance that largely stays the same between all animals.

All have very soft and fragile bodies, and all have the similar division of three parts making up their body being the proboscis, collar and trunk regions.

Each species has a body wall with a single layered epidermis, along with gill slits commonly used for the feeding process which can be one or numerous.

The proboscis is important for most hemichordata species because it not only secretes the tubular coenecium system in which most of them reside in, it also helps guide them through the tube itself and are great for burrowing by extending the proboscis forward and using it in an anchor type motion.

The collar region is mostly used for feeding since it is comprised of the creatures mouth, coelom, stomochord and for pterobranchs, it also includes ciliated tentacles which are important for generating small water currents and using them to capture and trap food particles that come their way.

The trunk houses the gonads, sac-like organs that produce bulges and are located between the genital ridges and the gill slits.

The trunk is the most elongated part of a Phylum Hemichordata animal composed of three regions: The brnchiogenital region, the posterior abdominal region and the hepatic caecal region.

The circulatory system of most hemichordates is usually quite simple but well developed, usually being made up of a contractile heart vessel with two extra longitudinal vessels, one dorsal and one ventral, interconnected by lateral vessels and sinuses.

Their nervous system is fairly primitive, being composed mainly of an intraepidermal nerve plexus.

Fertilization is also external, with developments occurring mostly indirectly through a free swimming tornaria larva, however direct development has been noticed in some forms.

Life Cycle

In regards to enteropneusta or acorn worms, they begin their life as a fertilized egg which before long hatches into a planktonic torn aria larvae.

They are direcious meaning they have two separate sexes, and although they can perform asexual reproduction through a method of fission, more commonly the female will lay a large number of eggs embedded in the gelatinous mass of mucus.

Depending on the species, the number of eggs can range anywhere from 100 to a few thousand.

The larvae stage is also skipped entirely by some species, instead the eggs hatch to produce already juvenile worms that will grow into mature worms, however for most species the larvae stage will still be present.

For pterobranchs who form colonies of genetically identical animals, for most of them they repdocue asexually through budding.

Budding involves an individual developing from some generative anatomical point of the parent organism and while it can be done from essentially any part of the body, for most species it is restricted to particular specialized areas.

The initial protuberance of proliferating cytoplasm or cells, also known as the bud, soon grows and develops into an organism that will duplicate the parent.

This new individual in some species can separate to exist independently from the parent, however in the case of pterobranchs they will often stay attached to form part of the colony.

Because graptolites have been harder to analyse and reserach, their life cycle has been a bit harder to confirm and narrow down for scientists, however from what research has been conducted it seems probable that their life started through regular sexual production between opposite sexes.

Rhadopleura is one of the only living genus of graptolites and so the way they reproduce is probably quite similar to other species considered grapolites.

During this process, the colonies do possess male and female zooids living together, suggesting that sexual reproduction was the case rather than asexual, but fertilised eggs are only incubated in the female tubarium and they have been seen to stay there until they become larvae and are able to swim freely on their own which usually takes around a week.

Each new larva will cover itself in a protective cocoon as soon as it is born.

Common Phylum Hemichordata Species

With over 100 species now classed as phylum hemichordates and with this number only growing as more research is conducted, it can be useful to know about some of the most well analyzed and common types that are around today.

Saccoglossus

The Saccoglossus is one of the most well known species classed as an enteropneust. They are marine, slender and soft body worms that live in spiral shaped burrows normally.

Just the same as most other hemichordates, they have a proboscis which is very elongated, collar which for them slightly overhangs the trunk, and the trunk itself which is separated into the anterior and branchial region.

They are often considered the staple example of a species belonging to the enteropneust class and are often found in the Indo-Pacific Atlantic Coasts as well as New Zealand and Australia.

Cephalodiscida

A member of the pterobranchia class, these creatures or zooids occur together in large aggregations attached to objects in the depths of the sea.

They follow the general habit of most pterobranchs living in a common matrix tunnel system known as the coenecium, however as individuals they are technically separate from each other.

Each zooid has the same general structure consisting of a proboscis, collar and trunk, however they do stand out because of the bent disc shape their proboscis takes.

Like many pterobranchs, they also bear denticulated arms which help to drive food laden current of water into their mouth.

Rhabdopleura

One of the last living species classed as a grapolite, the rhabdopleura is fascinating because it is the closest we can get to an indication as to how other graptolites behaved when they were around centuries ago.

This species is usually found attaching itself to stones, corals and sessile marine animals and are often found in the North Atlantic.

They work in colonies within thin horizontal branching tubes known as the coenecium with short erect tubes that contain the separate zooids.

Each zooid is miniature in structure and is similarly equipped with a trunk, proboscis and collar.

What is particularly fascinating about rhadopleura and came as a big discovery regarding the behaviour of graptolites in general is that the sexes are separate with an individual being produced sexually and being hatched out of large yolky eggs that are often laid in bulk.

Summary

Species classed as being part of the Phylum Hemichordata can be categorized as marine animals similar in body shape and anatomy, however their specific characteristics, behaviors and life cycles can vary by quite a large degree depending on the variant.

While it now contains over 100 known species, new discoveries are being made concerning the classification of hemichordata creatures each year and with grapholites only recently being accepted as just one subset classed as a Hemichordata alongside Pterobranchia and Enteropneusta, the full characteristics, behaviors and qualities of this specific species group are constantly being refined and altered as the years go by.

Jennifer Dawkins

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