Information:  - Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like the oligochaetes, such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from the oligochaetes in significant ways. For example, leeches do not have bristles and the external segmentation of their bodies does not correspond with the internal segmentation of their organs. Their bodies are much more solid as the spaces in their coelom are dense with connective tissues. They also have two suckers, one at each end.  - Euhirudinea, the true leeches, are an infraclass of the Hirudinea. These clitellate annelids are of somewhat unclear relationships; namely the relationships of Hirudinea with oligochaetes are in need of revision. It may be that the presumed sister taxon of the Euhirudinea, the Acanthobdellidea, turns out to be more distantly related, as was already the case with the Branchiobdellida. Thus, eventually Euhirudinea might become a junior synonym of Hirudinea.  - The proboscisless leeches, Arhynchobdellida, are classified as an order of the Hirudinea. But leech taxonomy and systematics will eventually be revised in due time, not because there are many uncertainties about their phylogeny, but because the major clades of clitellate annelids - and whether the clitellates are themselves a clade - have not been fully elucidated. For example, the "true leeches" (Euhirudinea) might actually be synonymous with the Hirudinea, as all other leech-like annelids might not be very closely related to the true leeches.   - Anatomy. Form and function. Depending on the species, an adult earthworm can be from long and wide to long and over wide, but the typical "Lumbricus terrestris" grows to about long.  - In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which either partner can act as the "female" or "male". For example, the great majority of tunicates, pulmonate snails, opisthobranch snails and slugs are hermaphrodites. Hermaphroditism is also found in some fish species and to a lesser degree in other vertebrates. Most plants are also hermaphrodites.  - A clade (from , "klados", "branch") is a group of organisms that consists of a common ancestor and all its lineal descendants, and represents a single "branch" on the "tree of life".  - The Hirudiniformes are one of the currently - accepted suborders of the proboscisless leeches ( Arhynchobdellida ) . Their most well - known member is the European Medical Leech , Hirudo medicinalis , and indeed most of the blood - sucking `` worms '' as which leeches are generally perceived belong to this group . In general , though some leeches suck blood , many are predators which hunt small invertebrates . The Arhynchobdellida were formerly divided into two groups , denoted by presence or absence of toothed jaws . But this does not represent a natural division , as has now been determined - the most primitive proboscisless leeches are not found among the jawed blood - sucking forms as was generally believed , but among the jawless predators . `` Jawed leeches '' - termed `` Gnathobdellae '' or `` Gnathobdellida '' - are exclusively found among the Hirudiniformes , but the order contains a number of jawless families as well . The jawed , toothed forms make up the aquatic Hirudidae and the terrestrial Haemadipsidae and Xerobdellidae ( sometimes included in the preceding but worthy of recognition as an independent family ) . These might actually form a clade , which would then be placed at superfamily rank , but it seems that the Hirudidae might rather be close relatives of the carnivorous Haemopidae instead . Many of the most well - known leeches belong to this family , most notably the medical leeches , such as the European species , already mentioned , which is prominent among these . Other medical Hirudiniformes of lesser importance are for example other species of the genus Hirudo , the North American Medical Leech ( Macrobdella decora ) , and the Asian Medical Leech ( Hirudinaria manillensis ) . Among the better - known bloodsucking land leeches are the Indian Leech ( Haemadipsa sylvestris ) and the yamabiru or Japanese Mountain Leech ( Haemadipsa zeylanica ) .  - The clitellum is a thickened glandular and non-segmented section of the body wall near the head in earthworms and leeches, that secretes a viscid sac in which the eggs are deposited. It is present about behind the anterior end of the body (around the 14th, 15th and 16th segments). Its main function is to store the eggs of the worm.  - The coelom (, plural coeloms or coelomata ) (from "koîlos", hollow, cavity) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In developed animals, it is lined with a mesodermal epithelium. In other animals, such as molluscs, it remains undifferentiated.  - Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic trees, phylogenies). Phylogenies have two components, branching order (showing group relationships) and branch length (showing amount of evolution). Phylogenetic trees of species and higher taxa are used to study the evolution of traits (e.g., anatomical or molecular characteristics) and the distribution of organisms (biogeography). Systematics, in other words, is used to understand the evolutionary history of life on Earth.     After reading the paragraphs above, choose the best answer for the entity that related to 'hirudiniformes' with the relationship of 'instance of'.  Choices: - adult  - anatomy  - biology  - branch  - case  - clade  - fish  - function  - group  - history  - may  - order  - organism  - share  - single  - study  - systematics  - taxon  - taxonomy  - tract  - two  - wall
A:
taxon