第一讲　河蟹的生物学　　一、河蟹的外部形态和内部结构(一 )河蟹的外部形态1.头胸部和腹部河蟹体平扁 ,头部和胸部愈合成头胸部 ,腹部向后下方卷折 ,覆盖在头胸部和腹甲下方。体分 2 0节 ,其中头部 5节 ,胸部 8节 ,腹部 7节。河蟹头胸部背方外披几丁质甲壳 (外骨骼 ) ,称头胸 Lecture 1 crab biology First, the external shape and internal structure of crab (a) the crab’s external form 1 head and chest and crab body flat, head and chest heal head and chest, the abdomen under the roll back, Cover the head and chest under the plastron. Body 20 knots, of which 5 head, chest 8, 7 abdomen. Crab head and chest outside the chest covered with chitin (exoskeleton), said the head and chest