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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]).
Not all animals have neurons; Trichoplax and sponges lack nerve cells altogether. Neurons may be packed to form structures such as the brain of vertebrates or the neural ganglions of insects . The number of neurons and their relative abundance in different parts of the brain is a determinant of neural function and, consequently, of behavior.
The annelids (/ ˈ æ n ə l ɪ d z /), also known as the segmented worms, are animals that comprise the phylum Annelida (/ ə ˈ n ɛ l ɪ d ə /; from Latin anellus 'little ring'). [3] [a] The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches.
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).
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.
When a human or an animal, such as a dog or horse, is said to "have worms", it means that it is infested with parasitic worms, typically roundworms or tapeworms. Deworming is a method to kill off the worms that have infected a human or animal by giving anthelmintic drugs. "Ringworm" is not a worm at all, but a skin fungus.
Charles Darwin wrote of the interaction between size and complexity of invertebrate brains: . It is certain that there may be extraordinary activity with an extremely small absolute mass of nervous matter; thus the wonderfully diversified instincts, mental powers, and affections of ants are notorious, yet their cerebral ganglia are not so large as the quarter of a small pin's head.
Most turbellarians have pigment-cup ocelli ("little eyes"); one pair in most species, but two or even three pairs in others. A few large species have many eyes in clusters over the brain, mounted on tentacles, or spaced uniformly around the edge of the body.