似蚓蛔线虫俗称蛔虫，为寄生人体的肠道线虫中体形最大者，其成虫寄生于人的小肠，引起蛔虫病（Ascariasis)。
犬弓首线虫（Tosocaracanis)，简称犬蛔虫，是犬类常见的寄生虫，但其幼虫能在人体内移行，引起内脏幼虫移行症（Visceral larva migrans，VLM)。
一、形态
1. 成虫
长圆柱形，形似蚯蚓，活时成粉红色或微红色，死后为白色；体表有细横纹和两条明显的侧索；雌虫长约20-35cm，尾端钝圆，雄虫15-31cm，尾断向腹面卷曲。
2. 受精蛔虫卵
Freshly passed Ascaris eggs from faeces.
The eggs may appear from light to dark brown in color. Interference contrast. ×400. Enlarged by 5.4.
Ascaris eggs showing cleavage.
This process normally occurs in the soil and leads to the formation of the larva. Interference contrast. ×400. Enlarged by 5.4
Fertilised eggs of Ascaris dissected from the uterus.
As these eggs have not been exposed to intestinal contents no faecal debris is seen and the outer mammillated layer is completely intact. Interference contrast. ×400. Enlarged by 5.4.
大小约为45-75×35-50μm，宽椭圆形，（因与胆汁接触而)呈棕黄色，表面有一层波浪状的蛋白质膜，内含一大而圆的卵细胞，卵细胞与卵壳间有星月形空隙。
3. 未受精蛔虫卵
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left: Fertilised eggs of Ascaris dissected from the uterus. As these eggs have not been exposed to intestinal contents no faecal debris is seen and the outer mammillated layer is completely intact. Interference contrast. ×400. Enlarged by 5.4. right: Unfertilised eggs of Ascaris dissected from the uterus. Note the difference in shape and the disorganized contents interference contrast. ×400. Enlarged by 5.4. | |
长椭圆形，大小约为88-94×39-44μm，卵壳内含许多大小不等的折光性颗粒。
注意：若蛔虫卵的蛋白质膜脱落，卵壳则成无色透明，应注意与其他线虫卵的鉴别。
