Standard anatomical terms of location are used to describe unambiguously the anatomy of and other . The terms, typically derived from Latin or Greek language roots, describe something in its standard anatomical position. This position provides a definition of what is at the front ("anterior"), behind ("posterior") and so on. As part of defining and describing terms, the body is described through the use of and anatomical axes.
The meaning of terms that are used can change depending on whether a vertebrate is a biped or a quadruped, due to the difference in the neuraxis, or if an invertebrate is a non-bilaterian. A non-bilaterian has no anterior or posterior surface for example but can still have a descriptor used such as proximal or distal in relation to a body part that is nearest to, or furthest from its middle.
International organisations have determined vocabularies that are often used as standards for subdisciplines of anatomy. For example, Terminologia Anatomica, Terminologia Neuroanatomica, and Terminologia Embryologica for humans and Nomina Anatomica Veterinaria for animals. These allow parties that use anatomical terms, such as , , and , to have a standard set of terms to communicate clearly the position of a structure.
Introduction
Standard
anatomy terms of location have been developed, usually based on Latin and Greek words, to enable all biological and medical scientists,
,
Physician and
to precisely delineate and communicate information about animal bodies and their organs, even though the meaning of some of the terms often is context-sensitive.
Much of this information has been standardised in internationally agreed vocabularies for humans (
Terminologia Anatomica,
Terminologia Neuroanatomica, and
Terminologia Embryologica),
with
Nomina Anatomica Veterinaria and
Nomina Embryologica Veterinaria used for
zootomy.
Different terms are used for those that are bipedal and those that are quadripedal.
Standard anatomical position
Because
can change orientation with respect to their environment, and because
like limbs and
can change position with respect to the main body, terms to describe position need to refer to an animal when it is in its standard anatomical position, even when its appendages are in another position. This helps to avoid confusion in terminology when referring to the same animal in different postures.
In humans, this refers to the body in a standing position with arms at the side and palms facing forward.
In
quadrupeds this is an animal standing upright with all four feet on the ground and the head facing forward.
For a
fish this is belly down with neutral appendages.
Planes
[[File:Human and goat anatomical planes.jpg|thumb|The standard anatomical planes of both a human and a goat displaying three anatomical planes:
]]
Anatomical terms describe structures with relation to four main .
The four main planes are:
-
The median plane, also called the midsagittal plane, divides the body into left and right parts starting from the sagittal suture.
-
The , also called the parasagittal planes or paramedian planes, are planes that are parallel to the median plane.
-
The dorsal plane divides the body into dorsal (towards the backbone) and ventral (towards the belly) parts.
-
The transverse plane, also called the axial plane or horizontal plane, is perpendicular to the other two planes.
Sagittal planes and transverse planes are used as anatomical lines to delineate bodily regions. There are several transverse planes with clinical relevance in the division of the torso into sections. They include the transpyloric plane, the subcostal plane, and the transumbilical plane.
Axes
The three axes of a vertebrate, are formed in embryonic development before and during the
gastrulation stage.
Distinct ends of the embryo are chosen, and the axis is named according to those directions. The three main axes of a bilaterally symmetrical animal that intersect at right angles, are the left-right, the craniocaudal, and the anteroposterior axes.
-
The left-right axis, also known as the horizontal or frontal axis
-
The craniocaudal axis, also known as the rostrocaudal, longitudinal or cephalocaudal
-
The anteroposterior axis, also known as the dorsoventral, or sagittal axis
An organism that is round, or asymmetrical may have different axes.
Main terms
Superior and inferior
In the standard human anatomical position,
superior () or
cranial, describes something that is nearer to the head, and
inferior () or
caudal describes what is below, and nearer to the feet.
Examples are the superior mediastinum, and inferior mediastinum. Neuroanatomy examples are the superior colliculus, and the inferior colliculus.
In veterinary anatomy, the terms superior and inferior are not used except to describe the eye, eyelids, lips and inner ear, using instead
dorsal and
ventral.
Anterior and posterior
Anterior () describes what is in front, and
posterior () describes what is to the back of something.
For example, for many
fish the
gill openings are posterior to the eyes and anterior to the tail.
Dorsal and ventral
These two terms, used in veterinary anatomy, are also used in
human anatomy mostly in
neuroanatomy, and
embryology, to describe something at the back (dorsal, posterior) or front (ventral, anterior) of an organ, or organism.
The dorsal () surface, (also dorsum) of an organism or organ, refers to the back, or upper side, such as in the human, the dorsum of the tongue, the dorsum of the hand, and the dorsum of the foot. If talking about the skull, the dorsal side is the top.
The ventral () surface refers to the front, or lower side, of an organism, or organ such as the undersurface of the tongue.
In a fish, the dorsal fin is on the upper surface and its (pelvic fins) are on the belly or undersurface.
The terms are used in other contexts, for example in dorsal and ventral gun turrets on a bomber aircraft.
Medial and lateral
These terms describe how close something is to the median plane.
() describes something to the sides of an animal, as in "left lateral" and "right lateral".
Medial () describes structures close to the median plane, or closer to the median plane than another structure.
For example, in a human, the arms are lateral to the
torso. The
Sex organ are medial to the legs.
Temporal has a similar meaning to lateral but is restricted to the head.
The terms "left" and "right", or sinistral and dextral, refer to the halves of a bilaterally symmetrical body divided by the median plane.
Terms derived from lateral include:
-
Contralateral (): on the side opposite to another structure. For example, the right arm and leg are controlled by the left, contralateral, side of the brain.
-
Ipsilateral (): on the same side as another structure. For example, the left arm is ipsilateral to the left leg.
-
Bilateral (): on both sides of the body.
-
Unilateral () one-sided or single-sided: on one side of the body.
Varus deformity () and valgus deformity ( ) are terms used to describe angulation or bowing of a bone or joint within the coronal plane, where the distal portion deviates towards (varus) or away from (valgus) the midline.
Proximal and distal
The terms
proximal () and
distal () are used to describe parts of a feature that are close to or distant from the main mass of the body, respectively.
Thus the upper arm in humans is proximal and the hand is distal. The main mass is taken as the center, the
chest, or the
heart.
"Proximal and distal" are frequently used when describing , such as fins, , and limbs. Although the direction indicated by "proximal" and "distal" is always respectively towards or away from the point of attachment, a given structure can be either proximal or distal in relation to another point of reference. Thus the elbow is distal to a wound on the upper arm, but proximal to a wound on the lower arm.
This terminology is also employed in molecular biology and therefore by extension is also used in chemistry, specifically referring to the atomic loci of molecules from the overall moiety of a given compound.[
]
Rostral, cranial, and caudal
Specific terms exist to describe how close or far something is to the head or tail of an animal. To describe how close to the head of an animal something is, three distinct terms are used:
-
Rostral () describes something situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe.
-
Cranial () or cephalic () describes how close something is to the head of an organism.
-
Caudal () describes how close something is to the trailing end of an organism.
These terms are generally preferred in veterinary medicine and not used as often in human medicine.[Hickman, C. P. Jr., Roberts, L. S. and Larson, A. Animal Diversity. McGraw-Hill 2003 ][Miller, S. A. General Zoology Laboratory Manual McGraw-Hill, and ] For example, in , the eyes are caudal to the nose and rostral to the back of the head.
In humans, "cranial" and "cephalic" are used to refer to the skull, with "cranial" being used more commonly. The term "rostral" is rarely used in human gross anatomy and refers more to the front of the face than the superior aspect of the organism. But it is used in embryology, and neuroanatomy. Similarly, the term "caudal" is used more in embryology and neuroanatomy, and only occasionally in human gross anatomy.
Central and peripheral
Central and peripheral refer to the distance towards and away from the centre of something. That might be an organ, a region in the body, or an anatomical structure. For example, the central nervous system and the peripheral nervous systems.
Central () describes something at, or close to the centre.
Peripheral (, originally from Ancient Greek) describes something that is situated nearer to the body's surface, such as a peripheral nerve.
Superficial and deep
These terms refer to the distance of a structure from the surface.
Deep () describes something further away from the surface of the organism.
Superficial () describes something near the outer surface of the organism.
Combined terms
Many anatomical terms can be combined, either to indicate a position in two axes simultaneously or to indicate the direction of a movement relative to the body. For example,
anterolateral indicates a position that is both anterior and lateral to the body axis (such as the bulk of the
pectoralis major muscle), or to a named organ such as the anterolateral tibial tubercle.
The term can also describe the direction and location of something that enters or courses through the body such as the anterolateral system in the
spinal cord, and the anterolateral central arteries.
Another term
anteromedial is used for example in the anteromedial central arteries.
In the more internal brain and spinal cord of the central nervous system the terms dorsal and ventral and their combinations are often used in place of anterior and posterior. In these organs numerous references need to be used, and in the brain for example the prefrontal cortex has the divisions of the dorsomedial prefrontal cortex, and the dorsolateral prefrontal cortex. And the dorsomedial region has subcompartments that make use of other terms such as the anterior cingulate cortex, and infralimbic cortex. Structures such as the anterior cingulate cortex may be divided anatomically based on cognitive (dorsal), and emotional (ventral) components.
Proximodistal is the axis of an appendage such as an arm or a leg, taken from its tip at the distal part to where it joins the body at the proximal part.
In radiology, various X-ray views uses terminology based on where the X-ray beam enters and leaves the body, including the front to back view ( anteroposterior), the back to front view ( posteroanterior), and the side view ( lateral).
Modifiers
Several terms are commonly seen and used as
:
-
Sub- () is used to indicate something that is beneath, or something that is subordinate to or lesser than.
-
Hypo- () is used to indicate something that is beneath.
-
Infra- () is used to indicate something that is within or below. For example, the infraorbital nerve runs within the orbit.
-
Inter- () is used to indicate something that is between.
-
Super- or Supra- () is used to indicate something that is above something else.
-
Ab- (), and ad- () are used to indicate that something is towards (ad-) or away from (ab-) something else.
Other terms are used as , added to the end of words:
-
-al () For example femoral neck.
-
-ad (), equivalent to '-ally', is a suffix createing the adverb form to indicate that something moves towards (-ad) something else.
Other terms and special cases
Anatomical landmarks
The location of anatomical structures can also be described in relation to different anatomical landmarks used in anatomy,
surface anatomy, surgery, and radiology.
Structures may be described as being at the level of a specific vertebra, depending on the section of the vertebral column the structure is at.
References may also take origin from surface anatomy, made to landmarks that are on the skin or visible underneath.
Anatomical lines are theoretical lines, using either horizontal transverse planes, or vertical sagittal planes, used to describe anatomical location. For examples, the mid-clavicular line is used as part of the cardiac examination to feel the apex beat of the heart, and the axillary lines are reference lines for the axilla region. Other types of lines in anatomy include the curved nuchal lines on the occipital bone, and the gluteal lines on the ilium.
Mouth and teeth
Special terms are used to describe the mouth and teeth.
Fields such as
osteology,
paleontology and
dentistry apply special terms of location to describe the mouth and teeth. This is because although teeth may be aligned with their main axes within the jaw, some different relationships require special terminology as well; for example, teeth also can be rotated, and in such contexts terms like "anterior" or "lateral" become ambiguous.
For example, the terms "distal" and "proximal" (or "mesial") are used for surfaces of individual teeth relative to the midpoint of the
dental arch, and "medial" and "lateral" are used in the standard sense relative to the median plane.
Terms used to describe structures include "buccal" () and "palatal" () referring to structures close to the
cheek and
hard palate respectively.
Hands and feet
Several anatomical terms are particular to the hands and feet.
Additional terms may be used to avoid confusion when describing the surfaces of the hand and what is the "anterior" or "posterior" surface. The term "anterior", while anatomically correct, can be confusing when describing the
Hand of the hand; Similarly is "posterior", used to describe the back of the hand and arm. This confusion can arise because the forearm can
pronate and
supinate and flip the location of the hand. For improved clarity, the directional term
palmar () is commonly used to describe the front of the hand, and
dorsal is the back of the hand. The
palmar fascia is
palmar to the
of muscles which flex the fingers, and the dorsal venous arch is so named because it is on the dorsal side of the foot.
In humans, volar can also be used synonymously with palmar to refer to the palm of the hand, and can also be used to refer to the sole of the foot.
Similarly, in the limbs for clarity, the sides are named after the bones. In the forearm, structures closer to the radius are radial, structures closer to the ulna are ulnar, and structures relating to both bones are referred to as radioulnar, such as the distal radioulnar joint.
Rotational direction
Anteversion and
retroversion are complementary terms describing an anatomical structure that is rotated forwards (towards the front of the body) or backwards (towards the back of the body), relative to some other position. They are particularly used to describe the curvature of the
uterus.
-
Anteversion () describes an anatomical structure being tilted further forward than normal, whether pathologically or incidentally.
-
Retroversion () describes an anatomical structure tilted back away from something.
Other directional terms
Several other terms are also used to describe location. These terms are not used to form the fixed axes. Terms include:
-
Axial (): around the central axis of the organism or the extremity.
-
Luminal (): on the—hollow—inside of an organ's lumen (body cavity or tubular structure);
-
Terminal () at the extremity of a usually projecting structure; forming the end of a structure such as an axon terminal.
-
Visceral (): associated with the innermost layer of an organ within the body. For example, the visceral pleura covering the , contrasted with the parietal pleura lining the thoracic cavity.
-
Parietal (): pertaining to the wall of a body cavity as the parietal pleura lining the thoracic cavity, contrasted with visceral pleura.
-
Aboral (away from oral) is used to denote a location in an organism that is further from the mouth.
Other animals and organisms
Different terms are used because of different
in animals, whether animals stand on two or four legs, and whether an animal is symmetrical or asymmetrical. For example, as humans are bilaterally symmetrical, anatomical descriptions usually use the same terms as those for other vertebrates.
However, the standard human anatomical position means that their anterior/posterior and ventral/dorsal directions are the same, so the inferior/superior directions are used due to longstanding tradition instead of cranial/caudal, which apply regardless of position, as in other species.
The term "rostral" used to refer to the beak or nose in some animals is used less frequently in humans, with the exception of parts of the brain;
while humans do not have a visible tail (the coccygeal vertebrae are present and commonly called the "tailbone") the term "caudal" that refers to the tail-end is also sometimes used in humans and animals without tails to refer to the hind part of the body.
and other flatfish which lie on the seabed on their left or right side are asymmetric, with both eyes on the 'up' side, making anatomical nomenclature a challenge.
Invertebrates have a large variety of body shapes that can present a problem when trying to apply standard directional terms. Depending on the organism, some terms are taken by analogy from vertebrate anatomy, and appropriate novel terms are applied as needed. Some such borrowed terms are widely applicable in most invertebrates; for example proximal, meaning "near" refers to the part of an appendage nearest to where it joins the body, and distal, meaning "standing away from" is used for the part furthest from the point of attachment. In all cases, the usage of terms is dependent on the body plan of the organism.
Non-bilaterian organisms
In non-bilaterian organisms with a changeable shape, such as amoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed. Similarly, in radially symmetrical organisms, there is nothing to distinguish one line through the centre of the organism from any other. An indefinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be useless, as nothing would distinguish a chosen triad from any others. In such organisms, only terms such as superficial and deep, or sometimes proximal and distal, are usefully descriptive.
Elongated organisms
In organisms that maintain a constant shape and have one dimension longer than the other, at least two directional terms can be used. The long or longitudinal axis is defined by points at the opposite ends of the organism. Similarly, a perpendicular transverse axis can be defined by points on opposite sides of the organism. There is typically no basis for the definition of a third axis. Usually such organisms are planktonic (free-swimming) protists, and are nearly always viewed on , where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome or other unique structure is present.
Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome in Paramecium or Stentor), or the end that usually points in the direction of the organism's locomotion (such as the end with the flagellum in Euglena), is normally designated as the anterior end. The opposite end then becomes the posterior end.
Organisms that are attached to a substrate, such as Porifera and Protista also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (), whereas the end furthest from the attachment is referred to as the apical end ().
Radially symmetrical organisms
Radial symmetry include those in the group Radiata primarily (jellyfish, sea anemones and corals, and the Ctenophora).
Cnidarians have an incomplete digestive system, meaning that one end of the organism has a mouth, the oral end (), and the opposite aboral end () has no opening from the gut (coelenteron).
Comb jellies have a biradial symmetry about only two planes, a tentacular plane, and a pharyngeal plane.
Image:Radiate Radial Axes.JPG| Aurelia aurita, another species of jellyfish, showing multiple radial and medio-peripheral axes
File:Porania_pulvillus.jpg|The sea star Porania pulvillus, aboral and oral surfaces
Spiders
Special terms are used for spiders. Two such terms are useful in describing views of the legs and of spiders, and other . Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body..]]
, with PME being the largest]]
the AME are the largest.]]
See also
-
Chirality
-
Geometric terms of location
-
Reflection symmetry
![are asymmetric, with both eyes lying on the same side of the head.]] Non-bilaterian organisms In non-bilaterian organisms with a changeable shape, such as amoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed. Similarly, in radially symmetrical organisms, there is nothing to distinguish one line through the centre of the organism from any other. An indefinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be useless, as nothing would distinguish a chosen triad from any others. In such organisms, only terms such as superficial and deep, or sometimes proximal and distal, are usefully descriptive. Elongated organisms In organisms that maintain a constant shape and have one dimension longer than the other, at least two directional terms can be used. The long or longitudinal axis is defined by points at the opposite ends of the organism. Similarly, a perpendicular transverse axis can be defined by points on opposite sides of the organism. There is typically no basis for the definition of a third axis. Usually such organisms are planktonic (free-swimming) protists, and are nearly always viewed on , where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome or other unique structure is present. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome in Paramecium or Stentor), or the end that usually points in the direction of the organism's locomotion (such as the end with the flagellum in Euglena), is normally designated as the anterior end. The opposite end then becomes the posterior end. Properly, this terminology would apply only to an organism that is always planktonic (not normally attached to a surface), although the term can also be applied to one that is sessile (normally attached to a surface).James W. Valentine (2025). 9780226845487, University of Chicago Press. ISBN 9780226845487 Organisms that are attached to a substrate, such as Porifera and Protista also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (), whereas the end furthest from the attachment is referred to as the apical end (). Radially symmetrical organisms Radial symmetry include those in the group Radiata primarily (jellyfish, sea anemones and corals, and the Ctenophora). (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Adult Echinodermata, such as starfish, , and others are also included, since they have a Radial symmetry having five discrete symmetric parts arranged around a central axis. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Echinoderm are not included, since they are bilaterally symmetrical. Cnidarians have an incomplete digestive system, meaning that one end of the organism has a mouth, the oral end (), and the opposite aboral end () has no opening from the gut (coelenteron). They are radially symmetric around the oral-aboral axis. Having only the single distinctive axis, "lateral", "dorsal", and "ventral" have no meaning, and all can be replaced by the generic term peripheral (). Medial can be used, but in the case of radiates indicates the central point, rather than a central axis as in vertebrates. Thus, there are multiple possible radial axes and medio-peripheral (half-) axes. Comb jellies have a biradial symmetry about only two planes, a tentacular plane, and a pharyngeal plane. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Image:Radiate Radial Axes.JPG| Aurelia aurita, another species of jellyfish, showing multiple radial and medio-peripheral axes File:Porania_pulvillus.jpg|The sea star Porania pulvillus, aboral and oral surfaces Spiders Special terms are used for spiders. Two such terms are useful in describing views of the legs and of spiders, and other . Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body.B.J. Kaston (1972). 9780697048998, W.C. Brown Co.. ISBN 9780697048998 Most spiders have eight eyes in four pairs. All the eyes are on the carapace of the prosoma, and their sizes, shapes and locations are characteristic of various spider families and other taxon. (2025). 9780199734825, Oxford University Press, US. . ISBN 9780199734825 Usually, the eyes are arranged in two roughly parallel, horizontal and symmetrical rows of eyes. Eyes are labelled according to their position as anterior and posterior lateral eyes (ALE) and (PLE); and anterior and posterior median eyes (AME) and (PME). .]], with PME being the largest]] the AME are the largest.]] See also Chirality Geometric terms of location Reflection symmetry Categories: Animal Anatomy, Medical Terminology, Orientation, Position Page 1 of 1 1 Page 1 of 1 1](http://www.upcscavenger.com/wiki/Direcoes anatomicas.svg/){kind=link}
![are asymmetric, with both eyes lying on the same side of the head.]] Non-bilaterian organisms In non-bilaterian organisms with a changeable shape, such as amoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed. Similarly, in radially symmetrical organisms, there is nothing to distinguish one line through the centre of the organism from any other. An indefinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be useless, as nothing would distinguish a chosen triad from any others. In such organisms, only terms such as superficial and deep, or sometimes proximal and distal, are usefully descriptive. Elongated organisms In organisms that maintain a constant shape and have one dimension longer than the other, at least two directional terms can be used. The long or longitudinal axis is defined by points at the opposite ends of the organism. Similarly, a perpendicular transverse axis can be defined by points on opposite sides of the organism. There is typically no basis for the definition of a third axis. Usually such organisms are planktonic (free-swimming) protists, and are nearly always viewed on , where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome or other unique structure is present. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome in Paramecium or Stentor), or the end that usually points in the direction of the organism's locomotion (such as the end with the flagellum in Euglena), is normally designated as the anterior end. The opposite end then becomes the posterior end. Properly, this terminology would apply only to an organism that is always planktonic (not normally attached to a surface), although the term can also be applied to one that is sessile (normally attached to a surface).James W. Valentine (2025). 9780226845487, University of Chicago Press. ISBN 9780226845487 Organisms that are attached to a substrate, such as Porifera and Protista also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (), whereas the end furthest from the attachment is referred to as the apical end (). Radially symmetrical organisms Radial symmetry include those in the group Radiata primarily (jellyfish, sea anemones and corals, and the Ctenophora). (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Adult Echinodermata, such as starfish, , and others are also included, since they have a Radial symmetry having five discrete symmetric parts arranged around a central axis. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Echinoderm are not included, since they are bilaterally symmetrical. Cnidarians have an incomplete digestive system, meaning that one end of the organism has a mouth, the oral end (), and the opposite aboral end () has no opening from the gut (coelenteron). They are radially symmetric around the oral-aboral axis. Having only the single distinctive axis, "lateral", "dorsal", and "ventral" have no meaning, and all can be replaced by the generic term peripheral (). Medial can be used, but in the case of radiates indicates the central point, rather than a central axis as in vertebrates. Thus, there are multiple possible radial axes and medio-peripheral (half-) axes. Comb jellies have a biradial symmetry about only two planes, a tentacular plane, and a pharyngeal plane. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Image:Radiate Radial Axes.JPG| Aurelia aurita, another species of jellyfish, showing multiple radial and medio-peripheral axes File:Porania_pulvillus.jpg|The sea star Porania pulvillus, aboral and oral surfaces Spiders Special terms are used for spiders. Two such terms are useful in describing views of the legs and of spiders, and other . Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body.B.J. Kaston (1972). 9780697048998, W.C. Brown Co.. ISBN 9780697048998 Most spiders have eight eyes in four pairs. All the eyes are on the carapace of the prosoma, and their sizes, shapes and locations are characteristic of various spider families and other taxon. (2025). 9780199734825, Oxford University Press, US. . ISBN 9780199734825 Usually, the eyes are arranged in two roughly parallel, horizontal and symmetrical rows of eyes. Eyes are labelled according to their position as anterior and posterior lateral eyes (ALE) and (PLE); and anterior and posterior median eyes (AME) and (PME). .]], with PME being the largest]] the AME are the largest.]] See also Chirality Geometric terms of location Reflection symmetry Categories: Animal Anatomy, Medical Terminology, Orientation, Position](http://www.upcscavenger.com/wiki/Anatomical-directions-kangaroo.svg/){kind=link}
![are asymmetric, with both eyes lying on the same side of the head.]] Non-bilaterian organisms In non-bilaterian organisms with a changeable shape, such as amoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed. Similarly, in radially symmetrical organisms, there is nothing to distinguish one line through the centre of the organism from any other. An indefinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be useless, as nothing would distinguish a chosen triad from any others. In such organisms, only terms such as superficial and deep, or sometimes proximal and distal, are usefully descriptive. Elongated organisms In organisms that maintain a constant shape and have one dimension longer than the other, at least two directional terms can be used. The long or longitudinal axis is defined by points at the opposite ends of the organism. Similarly, a perpendicular transverse axis can be defined by points on opposite sides of the organism. There is typically no basis for the definition of a third axis. Usually such organisms are planktonic (free-swimming) protists, and are nearly always viewed on , where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome or other unique structure is present. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome in Paramecium or Stentor), or the end that usually points in the direction of the organism's locomotion (such as the end with the flagellum in Euglena), is normally designated as the anterior end. The opposite end then becomes the posterior end. Properly, this terminology would apply only to an organism that is always planktonic (not normally attached to a surface), although the term can also be applied to one that is sessile (normally attached to a surface).James W. Valentine (2025). 9780226845487, University of Chicago Press. ISBN 9780226845487 Organisms that are attached to a substrate, such as Porifera and Protista also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (), whereas the end furthest from the attachment is referred to as the apical end (). Radially symmetrical organisms Radial symmetry include those in the group Radiata primarily (jellyfish, sea anemones and corals, and the Ctenophora). (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Adult Echinodermata, such as starfish, , and others are also included, since they have a Radial symmetry having five discrete symmetric parts arranged around a central axis. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Echinoderm are not included, since they are bilaterally symmetrical. Cnidarians have an incomplete digestive system, meaning that one end of the organism has a mouth, the oral end (), and the opposite aboral end () has no opening from the gut (coelenteron). They are radially symmetric around the oral-aboral axis. Having only the single distinctive axis, "lateral", "dorsal", and "ventral" have no meaning, and all can be replaced by the generic term peripheral (). Medial can be used, but in the case of radiates indicates the central point, rather than a central axis as in vertebrates. Thus, there are multiple possible radial axes and medio-peripheral (half-) axes. Comb jellies have a biradial symmetry about only two planes, a tentacular plane, and a pharyngeal plane. (2025). 9780030259821, Thomson-Brooks/Cole. ISBN 9780030259821 Image:Radiate Radial Axes.JPG| Aurelia aurita, another species of jellyfish, showing multiple radial and medio-peripheral axes File:Porania_pulvillus.jpg|The sea star Porania pulvillus, aboral and oral surfaces Spiders Special terms are used for spiders. Two such terms are useful in describing views of the legs and of spiders, and other . Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body.B.J. Kaston (1972). 9780697048998, W.C. Brown Co.. ISBN 9780697048998 Most spiders have eight eyes in four pairs. All the eyes are on the carapace of the prosoma, and their sizes, shapes and locations are characteristic of various spider families and other taxon. (2025). 9780199734825, Oxford University Press, US. . ISBN 9780199734825 Usually, the eyes are arranged in two roughly parallel, horizontal and symmetrical rows of eyes. Eyes are labelled according to their position as anterior and posterior lateral eyes (ALE) and (PLE); and anterior and posterior median eyes (AME) and (PME). .]], with PME being the largest]] the AME are the largest.]] See also Chirality Geometric terms of location Reflection symmetry Categories: Animal Anatomy, Medical Terminology, Orientation, Position](http://www.upcscavenger.com/wiki/Anatomical Directions and Axes.JPG/){kind=link}
![are asymmetric, with both eyes lying on the same side of the head.]]](http://www.upcscavenger.com/wiki/Horse Axes.JPG/){kind=link}