在低等动物当中,蚯蚓可说是独具特色的一个物种了。它是除昆虫幼虫之外,陆地上最常见的蠕虫,是专在土壤中生活的小动物。它没有明显的头部与躯干部之分,也无任何肢体,全身由许多环节组成。蚯蚓属于环节动物门寡毛纲,比蛔虫、绦虫等寄生虫高级,但比节肢动物、软体动物、棘皮动物低级。蚯蚓前端有一个简单的口,后端是一个简单的肛门,除 Among the low-level animals, we can say that it is a unique species. It is the most common worm on land except for insect larvae. It is a small animal that lives exclusively in the soil. It has no obvious distinction between the head and the trunk, and it has no limbs. The body consists of many links. Cockroaches belong to the oligochaetes of the annelids, which are more advanced than aphids, aphids and other parasites, but lower in grade than arthropods, mollusks, and echinoderms. The front end has a simple mouth, the back end is a simple anus, except