The head of an Arrow Worm, possibly Sagitta elegans. The labels show the position of the teeth and spines that grab prey.
Worms that are Not Worms
Examples here: Arrow Worms - Acorn Worms - Ribbon Worms
The term "worm" is extremely vague but extensively used to describe long, narrow organisms. The only "true" worms are found in the phylum Annelida, with marine species exclusively in the class Polychaeta.
Only 120 arrow-worms exist worldwide but are so weirdly different to anything else they get their own phylum, the Chaetognatha - "bristle mouths" effectively.
Another small phylum is the Hemichordata distantly related to the arrow-worms and are very worm-like. Originally they were included in the Chordate phylum because it was thought they had a notochord (see also the Tunicates).
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This section looks first at arrow worms and acorn worms. The last section is the phylum Nemertea, the Ribbon Worms.
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Arrow Worms - Chaetognatha
Arrow worms are exclusively marine. In the Haven they tend to be small, around 500 microns to 1mm in length. Highly predacious they feed on copepods and other animals. Hunting they move extremely fast. In the open sea they can reach several centimetres in length. The photos view them from top or bottom but side on they are extremely thin and flat, an adaptation that reduces sinking in the water column.
Close up of the head of an Arrow Worm Sagitta sp.
Hemichordate - Class Enteroapneusta: Acorn Worms
This planktonic larva is called a tornaria and is a well developed, late larval stage. Only once have I seen one of these in the Haven, probably the species Balanoglossus. They are beautiful to watch under low magnification. This one was taken at x10. Books suggest they are rarely seen.
This acorn worm is an adult of Saccoglossus and they do not have a planktonic larva. Acorn worms live in sand and other sediments. I have only seen two in my lifetime and this one I photographed in Poole Harbour in 1989. The yellow part is the proboscis with the mouth at the base. Mucus streams across it picking up particulate matter to consume. Gill pores are in the red/brown section.
Phylum Nemertea - the Ribbon Worms
Ribbon worms are unsegmented organisms that relate to the flatworms except the nemerteans can grow to spectacular lengths. They are not exclusively marine but in the UK the rocky shore is the best place to find them. The species most frequently seen is the Bootlace Worm Lineus longissimus when it is coiled up in a dark mass in crevices of the lower shore. Unravelling it (trying not to break it) they can extend for many metres, up to 10 metres in fact. Fragmentation is a method of asexual reproduction.
Sexually they produce a planktonic larva, the pilidium. Helmet shaped with the mouth underneath it has two ciliated lobes that come down on either side. As the pilidium grows the juvenile worm forms within it rather than the larva turning into the next stage. Only the larval gut survives into the adult and the young nemertean that emerges from the pilidium will often eat the remnants of the larva. All quite bizarre and unusual.
Over the years I have found the occasional specimen but they have never been a common sight. Also the shape varies and is not thought to be species specific.
Bootlace Worm Lineus longissimus
Quick Links:
Arrow & Acorn Worms
Bryozoa & Phoronids
Crustacea 1 - Copepods
Crustacea 2 - Larval Crab & Barnacle
Diatoms
Echinoderms
Foramens
Jellies - Cnidarians & Ctenophores
Molluscs
Parasites - flatworms & crustacea
Polychaetes - marine worms
Protists 1 - Dinoflagellates
Protists 2 - Ciliates
Rotifers
Tunicates
Decay, Oddities and Unknowns
