Acetabulum (: acetabula) in

invertebrate zoology Invertebrate zoology is the subdiscipline of zoology that consists of the study of invertebrates, animals without a backbone (a structure which is found only in fish, amphibians, reptiles, birds and mammals). Invertebrates are a vast and very dive ...

is a

saucer A saucer is a type of small dishware. While in the Middle Ages a saucer was used for serving condiments and sauces, currently the term is used to denote a small plate or shallow bowl that supports a cup – usually one used to serve coffee t ...

-shaped organ of attachment in some annelid worms (like

leech Leeches are segmented parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...

) and

flatworms Platyhelminthes (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates commonly called ...

. It is a specialised sucker for

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...

in trematodes by which the worms are able to attach on the

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County * Host Island, in the Wilhelm Archipelago, Antarctica People * ...

. In annelids, it is basically a locomotory organ for attaching to a substratum. The name also applies to the suction

appendage An appendage (or outgrowth) is an external body part or natural prolongation that protrudes from an organism's body such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

on the arms of cephalopod molluscs such as

squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

octopus An octopus (: octopuses or octopodes) is a soft-bodied, eight-limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like oth ...

cuttlefish Cuttlefish, or cuttles, are Marine (ocean), marine Mollusca, molluscs of the order (biology), suborder Sepiina. They belong to the class (biology), class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique ...

, ''

Nautilus A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina. It comprises nine living species in two genera, the type genus, ty ...

'', etc.

Etymology

Acetabulum literally means "a small saucer for

vinegar Vinegar () is an aqueous solution of diluted acetic acid and trace compounds that may include flavorings. Vinegar typically contains from 5% to 18% acetic acid by volume. Usually, the acetic acid is produced by a double fermentation, converting ...

". It is derived from two

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

words ''acetum'', meaning "vinegar", and ''-bulum'', a

suffix In linguistics, a suffix is an affix which is placed after the stem of a word. Common examples are case endings, which indicate the grammatical case of nouns and adjectives, and verb endings, which form the conjugation of verbs. Suffixes can ca ...

denoting "saucer" or "vessel" or "bowl". The name is used because of the saucer-like structure in the invertebrates.

Structure

Annelids

In leeches, acetabulum refers to the prominent posterior sucker at the extreme end of the body. In fact it forms a head-like structure, while the actual head is relatively small. It is a thick disc-shaped muscular system composed of circular, longitudinal and radial fibers.

Trematode

In flatworms, acetabulum is the ventral sucker situated towards the anterior part of the body, but behind the anterior oral sucker. It is composed of numerous spines for penetrating and gripping the host tissue. The location and structure of the acetabulum, and the pattern of the spine alignment are important diagnostic tool among trematode species.

Mollusc

Acetabulum in molluscs is a circular hollow opening on the

arms Arms or ARMS may refer to: *Arm or arms, the upper limbs of the body Arm, Arms, or ARMS may also refer to: People * Ida A. T. Arms (1856–1931), American missionary-educator, temperance leader Coat of arms or weapons *Armaments or weapons **Fi ...

. It occupies the central portion of the sucker and surrounded by a larger spherical cavity infundibulum. Both these structures are thick muscles, and the acetabulum is specifically composed of radial muscles. They are covered with

chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...

ous cuticle to make a protective surface.

Function

Acetabulum is essentially an organ of attachment. In annelids, it is used for adherence to the substratum during a looping locomotion. Annelid worms such as leeches move by repeated alternating extensions and shortenings of the body. This in turn is done by successive attachment and detachment of the oral sucker and the acetabulum. In flukes it is used for penetrating the mucosal wall of the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the Digestion, digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascula ...

for maintaining its parasitic habitat. It is sensory in nature consisting of type 2

sensory receptor Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduc ...

, which is a smooth bulb-like non-ciliated papilla.

Mollusc

Molluscans uses it for grasping substratum, catching prey and for locomotory accessory. The best studied acetabular activity is that of octopus. Octopus arms contains 200-300 independently controlled suckers that can grasp small objects and produce high adhesion forces on virtually any non-porous surface. This precise mechanism of high flexibility even has a potential mechanical applications in

robotics Robotics is the interdisciplinary study and practice of the design, construction, operation, and use of robots. Within mechanical engineering, robotics is the design and construction of the physical structures of robots, while in computer s ...

. Each sucker is a

tactile sensor A tactile sensor is a device that measures information arising from physical interaction with its environment. Tactile sensors are generally modeled after the biological sense of cutaneous receptor, cutaneous touch which is capable of detect ...

for detecting the surrounding. When the sucker attaches itself on an object, the infundibulum mainly provides adhesion while the central acetabulum is quite free. This provides greater suction on the flat surface; hence, making pressure incredibly low. This is why octopus grip is exceptionally firm. Then contraction of the radial muscle of the acetabulum causes detachment of the entire sucker.

References

External links

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Animal anatomy Animal morphology