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Free-living worm species do not live on land but instead live in marine or freshwater environments or underground by burrowing. In biology, "worm" refers to an obsolete taxon , Vermes , used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non- arthropod invertebrate animals , now seen to be paraphyletic .
Arthropoda is the largest animal phylum with the estimates of the number of arthropod species varying from 1,170,000 to 5~10 million and accounting for over 80 percent of all known living animal species. [36] [37] One arthropod sub-group, the insects, includes more described species than any other taxonomic class. [38]
Arthropods are intermediate hosts of Hymenolepis nana, otherwise known as the "dwarf tapeworm," while humans are used as final hosts. Humans become infected and develop hymenolepiasis through eating infected arthropods, ingesting eggs in water inhabited by arthropods, or from dirty hands. This is a common and widespread intestinal worm. [5]
Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (helminths) are the cause of soil-transmitted helminthiases. They are classified along with arthropods, tardigrades and other moulting animals in the clade Ecdysozoa.
Well-known phyla of invertebrates include arthropods, mollusks, annelids, echinoderms, flatworms, cnidarians, and sponges. The majority of animal species are invertebrates; one estimate puts the figure at 97%. [1] Many invertebrate taxa have a greater number and diversity of species than the entire subphylum of Vertebrata. [2]
There are over 22,000 living annelid species ... of arthropods' eyes. [23] Some tube-worms use ... over one-third of bottom-dwelling animal species ...
One or more pairs of spermathecae are present in segments 9 and 10 (depending on the species) which are internal sacs that receive and store sperm from the other worm during copulation. As a result, segment 15 of one worm exudes sperm into segments 9 and 10 with its storage vesicles of its mate.
More than 99 percent of all species, amounting to over five billion species, [7] that ever lived on Earth are estimated to be extinct. [8] [9] Estimates on the number of Earth's current species range from 10 million to 14 million, [10] of which about 1.2 million have been documented and over 86 percent have not yet been described. [11]