Introduction to Annelid Worms.
Oligochaete Worm Gallery.
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The Annelid Worms.
(Segmented worms, including Leeches).
Two classes of annelid worm are covered here:
Annelid worms are mostly macroscopic and free-living. Their bodies are cylindrical or flattened, and are divided into a number of segments which may be as few as a dozen or so, or more than a hundred. Most segments bear bundles of chaetae, or bristles -- numerous in many of the marine forms, or few in the case of the earthworm and the microscopic freshwater forms.
Included in this phylum are the leeches, the marine bristle worms, and the earthworms.
The annelids are usually divided into three classes: Oligochaete worms, Polychaete worms, and the Hirudinae, or leeches. The mostly marine Polychaete worms are not covered here.
The Oligochaete Worms.
This class includes a wide variety of worms, primarily freshwater and terrestrial in habitat, with giant earthworms having pigmented bodies and few chaetae at one end of the scale, and tiny colourless aquatic worms with pronounced chaetae at the other. The smaller aquatic forms are the ones shown in these galleries.
They are usually found in the mud and detritus of the pond bottom, and in the tangles of filamentous algae floating on the surface and anchored at the edges of the pond.
They are scavengers -- anything digestible is absorbed from the food intake during its passage through the intestine. In turn, the faecal pellets which fall to the floor of the pond, consisting of undigested remains loosely bound with mucous secretions from the gut of the worm, form a suitable substrate for the support of other life such as unicellular algae, bacteria, and the ciliates, rotifers and other worms which are normally found in their company.
Dero is a not particularly common oligochaete worm, but is a spectacular creature when encountered. The tail section is modified into a hand-like gill structure in which the "fingers" are completely covered with cilia. An unforgettable impression is created when these gills suddenly expand from the "clenched" state shown here.
This sequence of darkfield pictures (above, all x200) are taken along the length of a single specimen. The first picture shows the head with the mouth and pharynx. The worm feeds by sudden expansions of the muscular pharynx causing the rapid intake of water containing debris and food organisms which are then passed to the gut. Pictures 2 and 3 show sections of the body. No. 3 clearly shows many diatoms along with organic debris.
The tail segments are shown in picture 4, with the gill structures tightly contracted.
In the brightfield pictures (below) of the same specimen, no. 1 shows the partially expanded gills, and the remaining pictures show the head in profile at various moments in the feeding action.
These worms are voracious feeders, and examination of the gut along the length of the animal often reveals protozoans such as Coleps, diatoms and other algae, as well as small crustaceans and rotifers all in the process of digestion.