Monotremes - Tankourui (English spelling)

A general term for animals belonging to the order Monotremata (Mammalia). Unlike other living mammals (Theria), members of this order Monotremata (Prototheria) are oviparous, have no nipples, and have mammary glands that open into the mammary gland area on the ventral surface and have a cloacal outlet. They also lack a tricuspid valve between the right atrioventricular zone of the heart, have separate ribs in the neck, coracoid, precoracoid, and interclavicular bones in the pectoral girdle, and have no spines on the scapula. They are 40 to 80 centimeters long, covered in hair or needle-like hairs, and have traces of scales. They have short limbs, small eyes, small or absent ears, and a horny beak. They lack vocal cords and are incompletely homeothermic, with a body temperature of 25 to 36 degrees Celsius. They lay eggs surrounded by a soft shell, are polycheloneous, locally cleaved, and only the eggs from the left ovary develop. Males have a claw with a small hole on their heel that leads to a poison gland. No fossils are known. They are endemic to the Australian region, and there are two families with three to six species. (1) Echidna Family: The short-billed echidna and the long-billed echidna (several species) are found in Australia, Tasmania, and New Guinea. They are terrestrial, toothless, and feed on ants and termites. Females have a pouch on their abdomen in which they place and raise their eggs. (2) Platypus Family: Only the platypus is found in Australia and Tasmania. They are aquatic, and develop teeth at a young age. They dig holes in the water to make nests, where they lay their eggs. A stuffed specimen of the platypus arrived at the British Museum in London in 1798, but was thought to be a composite until 1802. It was not until 1884 that both the platypus and the echidna were discovered to be oviparous.

[Yoshinori Imaizumi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

哺乳(ほにゅう)綱単孔目に属する動物の総称。この目Monotremataの仲間（原獣亜綱）は、他の現生の哺乳類（獣亜綱）と異なり卵生で乳頭がなく、乳腺(にゅうせん)は腹面の乳腺区に開孔し、総排出孔がある。また、心臓の右側房室間に三尖弁(さんせんべん)がなく、頸(けい)部に独立した肋骨(ろっこつ)、胸帯に烏口(うこう)骨、前烏口骨、間鎖骨があり、肩甲骨に棘(きょく)突起がない。体長40～80センチメートル、体は毛または針状毛で覆われ、鱗(うろこ)の痕跡(こんせき)がある。四肢は短く、目が小さく、耳介はないか小さく、角質のくちばしがある。声帯がなく、不完全な恒温性で体温は25～36℃。卵は柔らかい殻に包まれ、多黄で局分割性、左の卵巣のものだけが発達する。雄のかかとには細い穴があいたけづめがあり、毒腺につながる。化石は知られていない。オーストラリア区の特産で、2科3～6種がある。(1)ハリモグラ科　オーストラリア、タスマニア、ニューギニア島にハリモグラとナガハシハリモグラ（数種）がある。陸生で歯がなくアリやシロアリを食べる。雌には腹部に育児嚢(のう)があり、卵をこの中に入れて育てる。(2)カモノハシ科　オーストラリアとタスマニアにカモノハシを産するのみである。水生で、幼時に歯が生える。水辺に穴を掘って巣とし、そこで卵を抱く。カモノハシの剥製(はくせい)標本は1798年にロンドンの大英博物館に到着したが、1802年まで合成物と思われていた。カモノハシとハリモグラが卵生とわかったのは、ともに1884年である。

［今泉吉典］

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