Other invertebrates

Freshwater sponges, like their marine counterparts, are colonial organisms. What we know as a sponge is the supporting structure which houses, and is built by, the cells making up the colony. Each cell has particular job to do. For instance, those in one group have whip-like extensions which are used to create a current which passes in and out of the sponge through its holes. This brings in oxygen, takes away waste, and carries microscopic particles which other specialised cells digest.

Hydra species are related to jellyfish and sea anemones, and look like the latter with their crowns of tentacles. They are very simple animals composed of only two layers of cells. Stinging cells on the tentacles shoot out poisonous barbs on the end of sticky threads to capture and paralyse tiny animals. These are then passed by the tentacles to the mouth, which is at their base. Movements of the body wall cause water to be drawn in or expelled, supplying oxygen and removing waste products respectively. Hydra attaches itself to a surface by its sticky base, but can move to another spot by "looping".

Parasitic flatworms include the various types of flukes and tapeworms, but there are also free-living species. These appear as little slimy blobs, often black in colour. They glide over the surface of stones, leaves etc., propelled by tiny beating hairs which cover their bodies, while the slime they produce helps them to slide along smoothly. Their diet consists of tiny animals mainly.

Somewhat more complex organisms than the flatworms are the roundworms, or nematodes, which are abundant in both soil and mud. A square metre of soil or mud may contain as many as twenty million of these, but they are often too small to notice. The nematodes include parasites of animals and plants, as well as free-living species which feed on tiny organisms.

The segmented worms, which include the familiar earthworms, marine ragworms and leeches, are the most advanced types. Earthworms do sometimes wander from the soil into the water, and provided that the water remains quite cool and well oxygenated they can survive quite happily in it. Their normal environment is, after all, a watery one, as they are bathed by the soil water; when the upper layers of soil dry out they must seek soil water lower down or die readily of desiccation.

Sludge worms, often known as Tubifex worms, are quite closely related to earthworms. Often they live in large colonies, with their heads stuck in the mud and their tails waving around. This waving creates a current to draw in fresh oxygen, which tends to be scarce in the bottom of the lake or pond. They are red in colour due to haemoglobin, the same blood pigment that carries oxygen in our own bodies, which helps them make the most of limited oxygen. They pass mud through their bodies and extract anything digestible from it.

Leeches share with earthworms and sludge worms the ringed appearance denoting the division of their bodies into segments. Mention the word "leech" and many people think of the tropical leeches which suck human blood. In Britain only the Medicinal Leech, which is now very rare, is capable of this. The other species pierce, and suck the insides out of, small creatures, or in some cases swallow them whole. A leech can swim, its action resembling that of an eel, or can loop its way across stones etc. using two round suckers, one on each end of its body.

Freshwater Sponge	Hydra
[Up to 200mm]	[Up to 30mm, most 4-10mm]
Flatworm	Roundworm (Nematode)
[0.5-35mm]	[Microscopic to 2-3mm]
Leech	Sludge Worm (Tubiflex)
[Up to 80mm]	[20-70mm]

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