Scrotum (English spelling)

Japanese: 陰囊 - いんのう（英語表記）scrotum

It is an organ unique to mammals, and is not found in primitive orders such as Monotremes, Insectivora, Odontida, and Squamata, nor in Proboscidea, marine Sirenia, or Cetacea. Furthermore, regardless of lineage, there is a tendency for scrotums to develop in the evolved members of each taxonomic group. For example, in the Perissodactyla, scrotums are absent in Tapiridae and Rhinocerosidae, but only in the Equidae, while in the Artiodactyla, they are not present in the primitive Suidae and Hippopotamidae, but are well developed in the highly evolved Cervidae, Giraffidae, and Bovidae. In Chiroptera, Dermoptera, and many rodents, scrotums form only during the breeding season and disappear at other times.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

哺乳類特有の器官で，単孔目，食虫目，貧歯目，有鱗目などの原始的な類にはなく，長鼻目，海生の海牛目，鯨目にもない。また系統とは無関係に，各分類群ごとに進化したものに陰囊が生ずる傾向が見られる。たとえば奇蹄目ではバク科，サイ科になく，ウマ科だけにあり，偶蹄目では原始的なイノシシ科，カバ科にはなく，進化度の高いシカ科，キリン科，ウシ科で発達している。翼手目，皮翼目，多くの齧歯（げつし）目では，陰囊は繁殖期にだけ形成され，他の時期には消失する。

出典　株式会社平凡社世界大百科事典第２版について　情報

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