Autotomy - Jisetsu

This refers to the phenomenon in which an animal amputates a part of its own body. It is usually a special case of a defense reflex or escape reaction, and refers to defensive autotomy that occurs when an animal is attacked by an external enemy or when a part of the body is seriously injured, but it can also include reproductive autotomy that occurs for reproduction or individual proliferation. In vertebrate animals, it is known that the tail of a lizard performs defensive autotomy, but in this case, several parts of the tail are easily amputated at the center of the vertebrae. Thus, in most cases, autotomy occurs at a specific location called the detachment segment. In arthropods such as crabs, autotomy due to a reflex (autotomy reflex) has developed in all walking legs, and detachment occurs at the surface where the base segment is fused with the seat segment. For the autotomy reflex to occur, excitation of a thick nerve that is generated by the coordinated action of at least two muscles and is not related to normal walking is required. Without this excitation, even if pulled with a strong external force, the detachment surface will not be destroyed, and it is clear that the detachment surface is not a mechanically weak place. On the other hand, some species, such as the arms of starfish and earthworms, lack a specific surface for detachment. Examples of reproductive autotomy are also found in many invertebrates, such as the coccyx of cephalopods and the proglottids of cestodes.

[Akira Murakami]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

動物が自ら体の一部を切断する現象をいう。通常は防衛反射または逃避反応の特殊な例として、外敵に襲われた場合または身体の一部に重大な傷害を受けた場合におこる防護自切をいうが、生殖または個体の増殖のため生じる生殖自切を含む場合がある。脊椎(せきつい)動物ではトカゲの尾が防護自切をすることが知られているが、この場合は尾の数か所が椎骨中央で切断しやすくなっている。このように、多くの場合、自切は脱離節とよばれる一定の場所でおこる。節足動物のカニの類では、反射（自切反射）による自切が全歩脚に発達し、基節が座節と癒合した面で脱離する。自切反射がおこるには、少なくとも二つの筋肉の協調した働きにより生じ通常の歩行には関係しない太い神経の興奮が必要である。この興奮なしには、外部から強い力で引いても脱離面では破壊されず、力学的に弱い場所が脱離面となるのではないことがわかる。一方、ヒトデの腕やミミズのように特定の脱離面がないものもある。また生殖自切の例も、頭足類の交接腕や条虫類の片節など多くの無脊椎動物にみられる。

［村上　彰］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例