Inzoology, atentacle is a flexible, mobile, and elongatedorgan present 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 taste of particular foods or threats. Examples of such tentacles are theeyestalks of 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. Anautilus has cirri, but asquid has 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.[1]
Some marine snails such asabalone and top snails,Trochidae, have numerous small tentacles around the edge of themantle. These are known aspallial tentacles.[2]
Amongcephalopods, squids 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".[3] These tentacles are made of stalks of axialnerve cords that 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.[1]
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.[2] 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 squid andcolossal squid have powerful suckers and pointed teeth at the ends. The teeth of the giant squid resemblebottle caps and function like tinyhole saws, while the tentacles of the colossal squid wield two long rows of swiveling, tri-pointed hooks.
Cnidarians, such asjellyfish,sea anemones,Hydra andcoral have numerous hair-like tentacles. Cnidarians have huge numbers ofcnidocytes on their tentacles. Inmedusoid form, the body floats on water so that the tentacles hang down in a ring around the mouth. Inpolyp form, such as sea anemone and coral, the body is below with the tentacles pointed upwards.
The tentacles of thelion's mane jellyfish may 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.[4][5]
Many species of the jellyfish-likectenophores have two tentacles, while some have none. Their tentacles have adhesive structures calledcolloblasts orlasso cells. The colloblasts burst open when prey comes in contact with the tentacle, releasing sticky threads that secure the food.[6]
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.[7]
Trypanorhynchcestodes are parasitic in fish. Theirscolex shows 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.[8]
The leglessamphibians calledcaecilians have 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.[2]
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 receptors calledEimer's organs, perhaps giving this mole the most delicate sense of touch among mammals.[2]
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 theCnidaria tentilla usually bearcnidocytes,[9] whereas in theCtenophora they usually havecollocytes.[10][11]Siphonophores are an example of Cnidaria that use tentilla.
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