Hemichordata
Introduction:
The hemichordates are a small, lesser known group that includes the acorn worms and sea angels. While previously thought to be the precursor group to the vertebrates (who fall into the group 'Chordata') it was later discovered that they are more closely related to the echinoderms.
They were considered closely related to vertebrates because of their 3 tissue layer structure, gill slits and stomochord running down the back of their body - which scientists mistook for the same organ as the spinal cord found in vertebrates. The stomochord has a different function despite appearing similar. It supports the heart and excretory organs in the 'collar' area of the body.
All hemichordates are marine dwellers and have existed for roughly 485 million years, with fossils dating to the Cambrian-Devonian period.
In the museum the hemichordates can be found in outer cabinet 10
Adaptations:
All hemichordates have a basic body plan consisting of a coelom body cavity, Proboscis (head), collar and trunk. The proboscis is equipped with many muscles and in some case specialist features to aid burrowing into marine sediment, where they feed and also shelter from predators.
They posses gill slits which they use for gas exchange to obtain oxygen and remove waste. Often they are also used for filter feeding to acquire nutrients from the surrounding waters and floating deposits.
Most hemichordates are covered in cilia- small hair like structures extending from cells. These aid with movement, gas exchange and feeding.
Some hemichordates are colonial organisms: made up of individual genetic clones (zooids) which are connected by tissue and sometimes a shared circulatory system. Each zooid has its own specialised function such as feeding, defence or reproduction.
Classification:
Hemichordates are divided into 3 classes
Enteropneusta
Solitary acorn worms
Pterobranchia
Colonial tube-building forms
Planctosphaeroidea
Only one species known by it's free-swimming larvae (adult form yet to be described)
Explore the hemichordate group in OneZoom
Museum Highlights:
A variety of acorn worm species featuring different shaped proboscis' adapted for digging in different sediments
Sea angel species - these pterobranchs are sessile, meaning they do not move, instead they attach to sediment or rocks where they obtain nutrients through filter feeding. They were given the name sea angel (not to be confused with the free-swimming slugs with the same name) based on their ethereal other worldly look
The worm-shaped Cephalodiscia class which live in clear water secreting tubes on the ocean floor
Acknowledgements:
Enteropneusta: By NOAA Photo Library - expn7526, Public Domain, https://commons.wikimedia.org/w/index.php?curid=65927277 Pterobranchia: By Adrian James Testa - https://collections.nmnh.si.edu/search/iz/?ark=ark:/65665/3db6c2545c10b444189b382fd11ec9e96, CC0, https://commons.wikimedia.org/w/index.php?curid=130977342 Planctosphaeroidea: Black Water Dive, 2021, Facebook, https://www.facebook.com/blackwaterdive/posts/2636943863218886/ C.densus: Michael Tassia, 2020, from: https://www.icyinverts.com/shipboard-blog/pterobranchs-oh-my