Tentacle

This article is about tentacles in zoology. For other uses, seeTentacle (disambiguation).

Cuttlefishwith two tentacles and eight arms

Inzoology,atentacleis a flexible, mobile, and elongatedorganpresent in some species ofanimals,most of theminvertebrates.In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mainly likemuscular hydrostats.Most forms of tentacles are used for grasping and feeding. Many aresensory organs,variously receptive totouch,vision,or to thesmell or tasteof particular foods or threats. Examples of such tentacles are theeyestalksof various kinds ofsnails.Some kinds of tentacles have both sensory and manipulatory functions.

A tentacle is similar to acirrus,but a cirrus is an organ that usually lacks the tentacle's strength, size, flexibility, or sensitivity. Anautilushas cirri, but asquidhas tentacles.

Invertebrates

Molluscs

See also:Cephalopod limb

Front view of land snail showing upper and lower sets of tentacles

Abalone showing pallial tentacles

Many molluscs have tentacles of one form or another. The most familiar are those of thepulmonate land snails,which usually have two sets of tentacles on the head: when extended the upper pair have eyes at their tips; the lower pair arechemoreceptors.Both pairs are fully retractablemuscular hydrostats,but they are not used for manipulation or prey capture. Molluscs have one pair of tentacles close to their mouths that hold close to their captured prey before they can consume it.

Some marine snails such asabaloneand top snails,Trochidae,have numerous small tentacles around the edge of themantle.These are known aspallial tentacles.

Amongcephalopods,squid have spectacular tentacles. They take the form of highly mobile muscular hydrostats with various appendages such as suction disks and sometimes thorny hooks. Up to the early twentieth century "tentacles" were interchangeably called "arms".These tentacles are made of stalks of axialnerve cordsthat are covered by circular transverse muscle tissue that contract in response to stimuli. There is a layer of helical muscle that helps each tentacle to twist or turn in any direction where the prey is sensed.

The modern convention, however, is to speak of appendages as "tentacles" when they have relatively thin "peduncles"or" stalks "with" clubs "at their tips. In contrast the convention refers to the relatively shorter appendages as" arms ". By this definition the eight appendages of octopuses, though quite long, count as arms.While arms are distinct from tentacles (a definition specific to the limb featuring peduncles), arms do fall within the general definition of "tentacle" as "a flexible, mobile, and elongated organ" and "tentacle" could be used as anumbrella term.

The tentacles of thegiant squidandcolossal squidhave powerful suckers and pointed teeth at the ends. The teeth of the giant squid resemblebottle capsand function like tinyhole saws,while the tentacles of the colossal squid wield two long rows of swiveling, tri-pointed hooks.

Cnidarians and ctenophores

Cnidarians,such asjellyfish,sea anemones,Hydraandcoralhave numerous hair-like tentacles. Cnidarians have huge numbers ofcnidocyteson their tentacles. Inmedusoid form,the body floats on water so that the tentacles hang down in a ring around the mouth. Inpolypform, such as sea anemone and coral, the body is below with the tentacles pointed upwards.

Many species of the jellyfish-likectenophoreshave two tentacles, while some have none. Their tentacles have adhesive structures calledcolloblastsorlasso cells.The colloblasts burst open when prey comes in contact with the tentacle, releasing sticky threads that secure the food.

The tentacles of thelion's mane jellyfishmay be up to 37 m (121 ft) long. They are hollow and are arranged in 8 groups of between 70 and 150. The longer tentacles are equipped with cnidocytes whose venom paralyses and kills prey. The smaller tentacles guide food into the mouth.

Bryozoa

Bryozoa(moss animals) are tiny creatures with tentacles around their mouths. The tentacles are almost cylindrical and have bands of cilia which create a water current towards the mouth. The animal extracts edible material from the flow of water.

Trypanorhynch cestodes

A larva oftrypanorhynchcestode (only two tentacles shown). Scale-bar: 0.1 mm

Detail of one tentacle with its spines. Scale-bar: 0.01 mm.

Trypanorhynchcestodesare parasitic in fish. Theirscolexshows four tentacles which are covered by spines. These tentacles help the adult cestode to attach to the intestine of the shark or ray that they parasitize. The same tentacles are also present in the larvae.

Vertebrates

Amphibians

The leglessamphibianscalledcaecilianshave two short tentacles, one on each side of the head, between their eyes and nostrils. The current opinion is that these tentacles supplement the normal sense of smell, possibly for navigation and to locate prey underground.

Mammals

Thestar-nosed mole,Condylura cristata,ofNorth America,has 22 short but conspicuous tentacles around its nose. They are mobile and extremely sensitive, helping the animal to find its way about the burrow and detect prey. They are about 1–4 mm long and hold about 25,000touch receptorscalledEimer's organs,perhaps giving this mole the most delicate sense of touch among mammals.

Tentillum

Deep-sea ctenophore trailing tentacles studded with tentilla

The wordtentillum(pl.:tentilla) literally means "little tentacle". However, irrespective of size, it usually refers to a side branch of a larger tentacle. In some cases, such tentilla are specialised for particular functions; for example, in theCnidariatentilla usually bearcnidocytes,whereas in theCtenophorathey usually havecollocytes.Siphonophoresare an example of Cnidaria that use tentilla.

External links

• TentacleArchived14 May 2013 at theWayback MachineatAbout
• Tentacleat theEncyclopædia Britannica
• The dictionary definition oftentacleat Wiktionary