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26-532: The Mesozoa are minuscule, worm -like parasites of marine invertebrates. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. A 2017 study recovered Mesozoa as a monophyletic group that emerged in the Lophotrochozoa as sister of the Rouphozoa . Some workers previously classified Mesozoa as the sole phylum of

52-587: A backbone) such as blindworms and caecilians . Worms include several groups. The three main phyla are: Familiar worms include the earthworms , members of phylum Annelida . Other invertebrate groups may be called worms, especially colloquially. In particular, many unrelated insect larvae are called "worms", such as the railroad worm , woodworm , glowworm , bloodworm , butterworm , inchworm , mealworm , silkworm , and woolly bear worm . Worms may also be called helminths , particularly in medical terminology when referring to parasitic worms , especially

78-411: A ciliated infusiform larva. There are three genera: Dicyema , Pseudicyema and Dicyemennea . Molecular evidence suggests that this phylum are derived from the Lophotrochozoa . Orthonectida are found in the body spaces of various marine invertebrates including tissue spaces, gonads, genitorespiratory bursae. This pathogen causes host castration of different species. The best known of Orthonectida

104-755: A few cells, but so far no muscle cells or neurons have been found in Dicyemida. The two main mesozoan groups are the Dicyemida and the Orthonectida . Other groups sometimes included in the Mesozoa are the Placozoa and the Monoblastozoa . Monoblastozoans consist of a single description written in the 19th century of a species that has not been seen since. As such, many workers doubt that they are

130-460: A long cylindrical tube-like body, no limbs , and usually no eyes . Worms vary in size from microscopic to over 1 metre (3.3 ft) in length for marine polychaete worms (bristle worms); 6.7 metres (22 ft) for the African giant earthworm, Microchaetus rappi ; and 58 metres (190 ft) for the marine nemertean worm (bootlace worm), Lineus longissimus . Various types of worm occupy

156-483: A pejorative epithet to describe a cowardly, weak or pitiable person. Worms can also be farmed for the production of nutrient-rich vermicompost . Limb (anatomy) All tetrapods have four limbs that are organized into two bilaterally symmetrical pairs, with one pair at each end of the torso , which phylogenetically correspond to the four paired fins ( pectoral and pelvic fins ) of their fish ( sarcopterygian ) ancestors. The cranial pair (i.e. closer to

182-464: A real group. As described, the animal had only a single layer of tissue. Rhombozoa, or dicyemid mesozoans, are found in the nephrid tracts of squid and octopuses . They range from a few millimeters long with twenty to thirty cells that include anterior attachment cells and a long central reproductive cell called an axial cell. This axial cell may develop asexually into vermiform juveniles or it may produce eggs and sperm that self-fertilize to produce

208-429: A small variety of parasitic niches, living inside the bodies of other animals. 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 . The name stems from

234-669: Is not a worm at all, but a skin fungus. Lobopodians are an informal grouping of extinct panarthropods from the Cambrian to the Carboniferous that are often called worms or "worm-like animals" despite having had legs in the form of stubby lobopods. Likewise, the extant Onychophora are sometimes called velvet worms despite possessing stubby legs. Wyrm was the Old English term for carnivorous reptiles ("serpents") and mythical dragons . "Worm" has also been used as

260-459: Is sometimes used to refer to parasitic worms. The term is more commonly used in medicine , and usually refers to roundworms and tapeworms. In taxonomy, "worm" refers to an obsolete grouping, Vermes , used by Carl Linnaeus and Jean-Baptiste Lamarck for all non- arthropod invertebrate animals , now seen to be polyphyletic . In 1758, Linnaeus created the first hierarchical classification in his Systema Naturae . In his original scheme,

286-531: Is the parasite of brittle stars. The multinucleate syncytial stage lives within tissues and spaces of the gonad but can spread into arms. It causes the destruction of starfish ovary and eggs to cause castration (the male gonads are usually unaffected). The stages of the plasmodium develop into more plasmodia by simple fragmentation; at some point, they decide to go sexual. The syncytia are dioecious (either male or female), but young syncytia can fuse to produce both male and female. The males are ciliated and smaller than

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312-467: The Nematoda (roundworms) and Cestoda (tapeworms). Hence, " helminthology " is the study of parasitic worms. 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 "

338-411: The Old English word wyrm . Most animals called "worms" are invertebrates , but the term is also used for the amphibian caecilians and the slowworm Anguis , a legless burrowing lizard . Invertebrate animals commonly called "worms" include annelids , nematodes , flatworms , nemerteans , chaetognaths , priapulids , and insect larvae such as grubs and maggots . The term " helminth "

364-462: The hand , has five opposable digits known as fingers (made up of metacarpal and metatarsal bones for hands and feet respectively) and specializes in intrinsic fine motor skills for precise manipulation of objects . The human legs and their extremities — the feet — are specialized for bipedal locomotion . Compared to most other mammals that walk and run on all four limbs , human limbs are proportionally weaker but very mobile and versatile, and

390-414: The head ) of limbs are known as the forelimbs or front legs , and the caudal pair (i.e. closer to the tail or coccyx ) are the hindlimbs or back legs . In animals with a more erect bipedal posture (mainly hominid primates , particularly humans ), the forelimbs and hindlimbs are often called upper and lower limbs , respectively. The fore-/upper limbs are connected to the thoracic cage via

416-413: The miracidium of trematodes , and their internal multiplication is similar to what happens in the sporocysts of trematodes. Mesozoan DNA has a low GC-content (40%). This amount is similar to ciliates, but ciliates tend to be binucleate. Others relate mesozoa to a group including annelids , planarians and nemerteans . Orthonectida have a very reduced muscular and nervous system, only consisting of

442-735: The pectoral/shoulder girdles , and the hind-/lower limbs are connected to the pelvis via the hip joints . Many animals, especially the arboreal species, have prehensile forelimbs adapted for grasping and climbing , while some (mostly primates) can also use hindlimbs for grasping. Some animals ( birds and bats ) have expanded forelimbs (and sometimes hindlimbs as well) with specialized feathers or membranes to achieve lift and fly . Aquatic and semiaquatic tetrapods usually have limb features (such as webbings ) adapted to better provide propulsion in water, while marine mammals and sea turtles have convergently evolved flattened, paddle -like limbs known as flippers . In human anatomy ,

468-510: The rib cage connected only via the clavicles (to the sternum ) and numerous muscles; while the pelvic girdles are typically fused together anteriorly via a fibrocartilaginous joint and posteriorly with the vertebral column ( sacrum ), forming an immobile ring-like pelvis . The girdles are each connected to the corresponding limb proper via a ball-and-socket synovial joint . The overall patterns of forelimbs and hindlimbs are homologous among all tetrapods, as they all branched out of

494-549: The 13th century, worms were recognized in Europe as part of the category of reptiles that consisted of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians", as recorded by Vincent of Beauvais in his Mirror of Nature . In everyday language, the term worm is also applied to various other living forms such as larvae , insects , millipedes , centipedes , shipworms (teredo worms), or even some vertebrates (creatures with

520-551: The animals were one of three kingdoms, divided into the classes of Vermes , Insecta , Pisces , Amphibia , Aves , and Mammalia . Since then the last four have all been subsumed into a single phylum, the Chordata , while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Lamarck, who called the Vermes une espèce de chaos (a sort of chaos) and split

546-461: The females. The females and the males leave the starfish and mate in the sea. Tailed sperm enters the female and fertilizes the numerous oocytes. Each oocyst produces a small ciliated larva which makes its way to another star. The genome of one of these species – Intoshia linei – has been sequenced. The Orthonectids may be degenerate annelid worms . Worm Worms are many different distantly related bilateral animals that typically have

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572-442: The group into three new phyla, worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique , Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes , annelids, crustaceans, arachnids, insects, worms, radiates , polyps, and infusorians . Chordates are remarkably wormlike by ancestry. In

598-575: The lonely subkingdom Agnotozoa . Cavalier-Smith argued that at least some of the mesozoans are in fact protistans , not animals. In the 19th century, the Mesozoa were a wastebasket taxon for multicellular organisms which lacked the invaginating gastrula which was thought to define the Metazoa . Mesozoa were once thought to be evolutionary intermediate forms between Protozoans and Metazoans , but now they are thought to be degenerate or simplified metazoa . Their ciliated larvae are similar to

624-466: The same bottlenecked lineage of stegocephalians that survived the Late Devonian extinction . The body plan of tetrapod limbs are so similar (especially the pentadactyly ) that they are given shared terminologies for each component of the appendicular skeleton . Limb development is controlled by Hox genes . All jawed vertebrates surveyed so far organize their developing limb buds in

650-404: The unique dexterity of the human upper extremities allows them to make sophisticated tools and machines that compensate for the lack of physical strength and endurance . Limbs are attached to the torso via girdles , either the pectoral girdle for the forelimbs , or the pelvic girdle for the hindlimbs . In terrestrial tetrapods, the pectoral girdles are more mobile, floating over

676-419: The upper and lower limbs are commonly known as the arms and legs respectively, although in academic usage, these terms refer specifically to the upper arm and lower leg (the lower arm and upper leg are instead called forearm and thigh , respectively). The human arms have relatively great ranges of motion and are highly adapted for grasping and for carrying objects . The extremity of each arm, known as

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