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Earthworms and their kin, in the subclass Oligochaeta, lack eyes but have photoreceptor cells in the skin, especially in the dorsal portion of the anterior end. They also lack parapodia and appendages on the prostomium, the body and the periproct (terminal segment on which the anus is located).
Earthworms have many internal parasites, including protozoa, platyhelminthes, mites, and nematodes; they can be found in the worms' blood, seminal vesicles, coelom, or intestine, or in their cocoons (e.g. the mite Histiostoma murchiei is a parasite of earthworm cocoons [53]).
Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them. Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from the circular muscles of the body. They are often supported internally by one or more large, thick chetae.
The remaining segments have no appendages, but they do have a small number of bristles, or chaetae. These tend to be longer in aquatic forms than in the burrowing earthworms, and can have a variety of shapes. Each segment has four bundles of chaetae, with two on the underside, and the others on the sides.
This parapodium is from a Museums Victoria specimen. Both lobes of the parapodium, the notopodium and neuropodium can bear chaetae. Chaetae on the notopodium are called notochaetae and those on the neuropodium are called neurochaetae. Thick, internal chaetae that provide support for well-developed notopodia or parapodia are called acicula.
Microscope photograph of a parapodium from a specimen of Arctonoe sp. showing the internal acicula that support the two lobes of the parapodium. This parapodium is from a Museums Victoria specimen. Most species of polychaete annelids have paired, fleshy parapodia which are segmentally arranged along the body axis. Parapodia vary greatly in size ...
Invasive earthworms can have a significant impact causing changes in soil profiles, nutrient and organic matter content and other soil organisms or plant communities. In most cases the disturbed areas includes agricultural systems or previously areas that are lacking of earthworms would see the biggest impact of the invasive earthworms. [ 28 ]
In earthworms, the clitellum can only be seen when the worm is sexually mature. It may be white, orange-red or reddish-brown in colour. Earthworms are ready to mate when their clitellum is orange. [2] In leeches, the clitellum appears during mating season, where it is used for both sexual reproduction and the secretion of a cocoon for the eggs. [4]