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Creatures from small phylum groups

Examples here: Arrow Worms and Acorn worms

The fine details of organism classification (taxonomy) is changing constantly and with our quest for order many creatures do not fit within existing systems. Strange anomalies often find themselves in a tiny phylum group of their own. 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). 

This section looks at two: arrow worms and acorn worms. Despite the name they are not even vaguely linked to the more primitive true worms (the Polychaetes).

Sagitta Arrow Worm Chaetognatha

The head of an Arrow Worm, possibly Sagitta elegans. The labels show the position of the teeth and spines that grab prey. 

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.

Full length Arrow Worm, Sagitta sp
Head view Arrow Worm, Sagitta sp
Head view Arrow Worm, Sagitta sp

Close up of the head of an Arrow Worm Sagitta sp. 

Sagitta, arrow worm head, alongside medusa tentacle

Hemichordate - Class Enteroapneusta: Acorn Worms

Tornaria larva of a Hemichordate, side view with copepod
Tornaria larva of a Hemichordate, labelled

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.

Hemichordate, Saccoglossus horsti from Poole Hbr

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. 

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