Crustaceans - Crustaceans

A group of animals that form the class Crustacea of ​​the phylum Arthropoda, and are represented by shrimp and crabs. They live mainly in water and breathe through gills, forming the majority of the Arthropods along with insects and spiders that breathe air through tracheas. The body is divided into the head, thorax, and abdomen, or the cephalothorax and abdomen. Each is divided into body segments, and in principle each has a pair of appendages. The appendages are functionally and morphologically differentiated according to the part of the body, and are called the first and second antennae, mandibles, and first and second mandibles in the head, the first to third maxillary legs, and the first to fifth swimming or walking legs in the thorax, and gastropods in the abdomen. The digestive tract runs straight from the mouth and divides into the oral passage, stomach, midgut, and hindgut, with the anus opening at the telson segment at the rear end. Some lower crustaceans retain the form of a primitive ladder-like nervous system, but in general, the left and right ganglia are united and centralized at the front and back. The circulatory system is an open vascular system, and in the absence of a heart, blood flow is generated by the movement of the body or digestive tract. The sinus gland, which is involved in the change of body color, and the male gland, which is involved in secondary sexual characteristics, are endocrine glands unique to crustaceans.

Fossils have been excavated from strata from the Cambrian period of the Paleozoic era onwards, but there are approximately 50,000 extant species, divided into the following nine subclasses.

(1) Cephalocarida: The most primitive crustaceans, with 4 genera and 9 species.

(2) Subclass Branchiopoda: Water fleas, etc.

(3) Ostracoda: Includes sea fireflies and other creatures with two shells.

(4) Copepoda: Plankton generally a few millimeters in length.

(5) Mystacocarida: A tiny species that lives in the sand of the intertidal zone, with only three species.

(6) Branchiura: Butterflies that parasitize the surface of fish.

(7) Subclass Cirripedia: Barnacles.

(8) Ascothoracica: A special group of animals that parasitize starfish and other organisms.

(9) Malacostraca: The so-called higher crustaceans, such as shrimp, crabs, and mantis shrimp.

[Masamichi Takeda]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

節足動物門甲殻綱Crustaceaを形成する動物群で、エビやカニ類などで代表される。主として水中にすみ、えらで呼吸し、気管によって空気呼吸をする昆虫類やクモ類とともに節足動物の主要部を形成する。体は頭部、胸部、腹部、あるいは頭胸部、腹部に分かれる。それぞれ体節に分かれ、原則的には1対ずつの付属肢をもつ。付属肢は体の部位によって機能的、形態的に分化し、頭部では第1および第2触角、大あご、第1および第2小あご、胸部では第1～第3顎脚(がっきゃく)、第1～第5遊泳脚あるいは歩脚、腹部では腹肢とよばれる。消化管は口から直走して、口道、胃、中腸、後腸に分かれ、後端の尾節に肛門(こうもん)が開く。下等甲殻類には原始的な梯子(はしご)状神経系の形態をとどめるものがあるが、一般に神経節の左右の合一や、前後の集中化が著しい。循環系は開放血管系で、心臓を欠く場合は体あるいは消化管の運動により血流をおこす。体色変化に関与するサイナス腺(せん)、第二次性徴に関与する雄性腺は甲殻類特有の内分泌腺である。

　化石は古生代カンブリア紀以降の地層に出土するが、現生種は約5万種で、次の9亜綱に分けられる。

(1)頭蝦亜綱(かしらえびあこう)Cephalocarida　もっとも原始的な甲殻類で、4属9種。

(2)鰓脚亜綱(さいきゃくあこう)Branchiopoda　ミジンコなど。

(3)貝形亜綱(かいけいあこう)Ostracoda　2枚の殻をもつウミホタルなど。

(4)橈脚亜綱(とうきゃくあこう)Copepoda　一般に体長数ミリメートルのプランクトン。

(5)鬚蝦亜綱(かみえびあこう)Mystacocarida　潮間帯の砂中にすむ微小種で、3種のみ。

(6)鰓尾亜綱(さいびあこう)Branchiura　魚類の体表に寄生するチョウ類。

(7)蔓脚亜綱(まんきゃくあこう)Cirripedia　フジツボ類。

(8)嚢胸亜綱(のうきょうあこう)Ascothoracica　ヒトデなどに寄生する特殊な動物群。

(9)軟甲亜綱Malacostraca　エビ、カニ、シャコなど、いわゆる高等甲殻類。

［武田正倫］

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