Gnathostomulid

Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. Most measure between 0.5 and 1 mm long. Like flatworms they have a ciliated epidermis, but are unique in having but one cilium per cell.[1] They have no body cavity, and no circulatory or respiratory system. Each gnathostomulid is simultaneously hermaphrodite, possessing an ovary and a testis. They are characterized by a specialized, muscular jaw, which they use to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat.[2] This bilaterally symmetrical pharynx with its complex cuticular mouth parts make them appear closely related to rotifers and their allies, together making up the Gnathifera.

There are approximately 100 described species and certainly many more as yet undescribed. The known species are grouped in two orders. The filospermoids are very long and are characterized by an elongate rostrum. The bursovaginoids have paired sensory organs and are characterized by the presence of a penis and a sperm-storage organ called a bursa.[2]

Gnathostomulids have no fossil record.

Hemichordata

Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the echinoderms. They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous. They seem to have a primitive form of notochord, but this is most likely the result of convergent evolution. A hollow neural tube exists among some species (at least in early life), probably a primitive trait they share with the common ancestor of chordata and the rest of the deuterostomes.

The Body of Hemichordates are divided into three parts, proboscis, collar and trunk. They have open circulatory systems also complete digestive tract but the musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia that cover its inside surface.

Hemichordata are divided into two classes: the Enteropneusta, commonly called acorn worms, and the Pterobranchia, which may include the graptolites. A third class, Planctosphaeroidea, is proposed based on a single species known only from larvae. The phylum contains about 100 living species. The exact taxonomic position of hemichordata and whether the group is monophyletic is currently under debate. One of the suggestions is that the pterobranchs are more basal deuterostomes, while the enteropneusts are an early offshoot of the lineage who are leading to Chordata.

Kinorhyncha

Kinorhyncha (Gr. kīneō 'move' + rhynchos 'snout') is a phylum of small (1 mm or less) marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons.

They are segmented, with a body consisting of a head, neck, and a trunk of eleven segments. They do not have external cilia, but instead have a number of spines along the body, plus up to seven circles of spines around the head,[1] which they use for locomotion, withdrawing the head and pushing forward, then holding with the spines while drawing up the body. The spines are part of a cuticle secreted by the epidermis; this is molted several times while growing to adulthood. The head is completely retractable, and is covered by a set of neck plates called placids when retracted.

Kinorhynchs eat diatoms and other things found in the mud.

There are two sexes that look alike, and the larvae are free-living, but little else is known of their reproductive process.

Their closest relatives are thought to be the phyla Loricifera and Priapulida. Together they constitute the Scalidophora.

The two groups of Kinorhynchs are still generally characterized as orders rather than classes, about 150 species are known.

Loricifera

Loricifera (from Latin, lorica, corselet + Greek, phora, bearing) is a small phylum of marine sediment-dwelling animals with twenty-two described species, in eight genera.[1][2] Aside from these described species, there are approximately 100 more which have been collected and not yet described.[1] They are characterised by a protective outer case called a lorica and their habitat, which is in the spaces between marine gravel to which they attach themselves. The phylum was discovered in 1983 by Reinhardt Kristensen, in Roscoff, France.[3] They are among the very latest of discovered groups of Metazoans.[4] They attach themselves quite firmly to the substratum, and so remained undiscovered for so long.[2] The first specimen was collected in the 1970s, and later described in 1983.[4] They are found at all depths, in different sediment types, and in all latitudes.[2]

The animals have a head, mouth and digestive system as well as a lorica. The armor-like lorica consists of a protective external shell or case of encircling plicae. There is no circulatory system and no endocrine system. Many of the larvae are acoelomate, with some adults being pseudocoelomate, and some remaining acoelomate.[4] The animals are hermaphrodites and probably oviparous. They have a very complex life cycle.[1] The species which live in the deep sea are able to reproduce via parthenogenetic or paedogenetic reproduction.[1] They are not known to be present in the fossil record.

Their closest relatives are thought to be the Kinorhyncha and Priapulida with which they constitute the taxon Scalidophora. The three phyla share four characters in common – chitinous cuticle, rings of scalids on the introvert, flosculi, and two rings of introvert retracts.[3][4] A group called Introverta is formed with Kinorhyncha, Priapulida, Nematoda, and Nematomorpha.

Mesozoa

The Mesozoa are enigmatic, minuscule, worm-like parasites. It is still unclear as to whether they are degenerate platyhelminthes (flatworms) or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. Decades ago, Mesozoa was classified as a phylum. But molecular phylogeny studies have shown that the mysterious mesozoans are polyphyletic. That is, they consist of two unrelated groups.[1]

As a result of these recent findings in molecular biology, the label mesozoan is now often applied informally, rather than as a formal taxon. Some workers previously classified Mesozoa as the sole phylum of the lonely subkingdom Agnotozoa.

Limnognathia

Limnognathia maerski is a microscopic animal, discovered living in homothermic springs on Disko Island, Greenland in 1994, that was given its own phylum, Micrognathozoa. It is related to the rotifers and gnathostomulids, grouped together as the Gnathifera. With an average length of one-tenth of a millimetre, it is one of the smallest animals known.

At first mistaken for a rotifer, on closer examination L. maerski was found to have significant differences in body structure from that of rotifers. These differences led the discoverers to assign L. maerski to its own phylum.

L. maerski has very complicated jaws, with fifteen separate elements. The parts of the jaw structure are connected by ligaments and muscles. The jaw parts are very small, ranging from 4 μm to 14 μm. The animal can extend part of its jaw structure outside of its mouth while eating. It also extends much of its jaw structure outside of its mouth when it is regurgitating items that are indigestible.

L. maerski has a large ganglion, or 'brain', in its head, and paired nerve cords extending ventrally (along the lower side of the body) towards the tail. Stiff sensory bristles made up of one to three cilia are scattered about the body. These bristles are similar to ones found on gnathostomulids, but up to three cilia may arise from a single cell in L. maerski, while gnathostomulids never have more that one cilia per cell.

Flexible cilia are arranged in a horseshoe shaped area on the forehead, and in spots on the sides of the head and in two rows on the underside of the body. The cilia on the forehead create a current that moves food particles towards the mouth. The other cilia move the animal.

All specimens of L. maerski that have been collected have had female organs. They lay two kinds of eggs: thin-walled eggs that hatch quickly, and thick-walled eggs that are believed to be resistant to freezing, and thus capable of over-wintering and hatching in the spring. The same pattern is known from rotifers, where thick-walled eggs only form after fertilization by males. The youngest L. maerski specimens collected may also have male organs, and it is now theorized that the animals hatch as males and then become females.

Nematode

The nematodes or roundworms (Phylum Nematoda from Greek νῆμα (nema): "thread" + -ώδη -ode "like") are one of the most common phyla of animals, with over 80,000 different described species (over 15,000 are parasitic). They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants, animals, and also in humans.

The Nematodes were originally named Nematoidea by Rudolphi (1808). They were renamed Nematodes by Burmeister 1837 (as a family; Leuckart 1848 and von Siebold 1848 both promoted them to the rank of order), then Nematoda (Diesing 1861), though Nathan Cobb (1919) argued that they should be called Nemata or Nemates (and in English 'nemas' rather than 'nematodes'). After some confusion which saw the nematodes placed (often together with the horsehair worms, Nematomorpha) as a class or order in various groups such as Aschelminthes, Lankester (1877) definitively promoted them to the level of phylum.

Nematomorpha

Nematomorpha (sometimes called Gordiacea, and commonly known as Horsehair worms or Gordian worms) are a phylum of parasitic animals which are morphologically and ecologically similar to nematode worms, hence the name. They are, on average, 1 meter long, and 1 to 3 millimetres in diameter. Horsehair worms can be discovered in damp areas such as watering troughs, streams, puddles, and cisterns. The adult worms are free living, but the larvae are parasitic on beetles, cockroaches, grasshoppers and crustaceans. About 320 species have been described.

Nematomorphs possess an external cuticle without cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems. Reproductively, they are dioecious, with the internal fertilization of eggs that are then laid in gelatinous strings.
Spinochordodes tellinii and its grasshopper host

In Spinochordodes tellinii, which has grasshoppers as its vector, the infection acts on the grasshopper's brain and causes it to seek water and drown itself, thus returning the nematomorph to water.[1] They are also remarkably able to survive the predation of their host, being able to wriggle out of the predator which has eaten the host cricket.[2]

Nemertea

Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms [1]. Most of the 1,400 or so species are marine, with a few living in fresh water and a small number of terrestrial forms; they are found in all marine habits, and throughout the world's oceans [2]. Nemerteans are named for Nemertes, one of the Nereids of Greek mythology, and alternative spellings for the phylum have included Nemertini, Nemertinea and Nemertea.

Velvet worm

Onychophora, the velvet worms (occasionally called walking worms or spitting worms), are segmented, caterpillar-like, terrestrial animals somewhat resembling both arthropods and annelid worms. They are generally regarded either as a class of arthropods or as a separate phylum (in which case they are the only animal phylum to include no known marine species). About 200 modern species are known separated into two families, with 10 genera — Peripatidae, found in many tropical and subtropical regions worldwide including Mexico, Central America, South America, and Africa, and Peripatopsidae, found in Chile, South Africa, Australia and New Zealand. Modern species are fairly small; at least one species reaches lengths of 20 cm (8 inches). The best known modern form is Peripatus which was described by Guilding in 1825.