一种寄生在水虱的腿上，另一种寄生在身体上。

One sort live on the creature's legs, the other on its body.

而且，刚才那个水中怪影的头部太大了，不像是一群水虱游在一起能形成的效果。

What's more, the head of that water monster was way too big to be only a large cluster of these insects swimming together.

就像寄生在人身上的三种体虱一样——它们是以抓住不同毛发的能力来分的——两种蛭形生物同样寄生在水虱身上。

Just as three sorts of body lice, specialised to be able to grasp different sorts of hair, live on humans, so water lice are host to two sorts of bdelloid.