Tentilla

In zoology, a tentacle is a flexible, mobile, elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mainly like muscular hydrostats. Most forms of tentacles are used for grasping and feeding. Many are sensory organs, variously receptive to touch, vision, or to the smell or taste of particular foods or threats. Examples of such tentacles are the "eye stalks" of various kinds of snails. Some kinds of tentacles have both sensory and manipulatory functions.

The word tentillum 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 the Cnidaria tentilla usually bear cnidocytes, whereas in the Ctenophora they usually have collocytes.

Many molluscs have tentacles of one form or another. The most familiar are those of the pulmonate 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 are chemoreceptors. Both pairs are fully retractable muscular hydrostats, but they are not used for manipulation or prey capture.

Some marine snails such as abalone and top snails, Trochidae, have numerous small tentacles around the edge of the mantle. These are known as pallial tentacles.

Among cephalopods, 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". 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. In terms of this article however, both types of organs count as tentacles because in relevant ways they are structurally and functionally similar.

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