I read a sentence in an article :

He ( Jules Verne) was ragged and cold. "My stockings," he told a friend,"are like a spiderweb in which a hippopotamus has been sleeping."

My question is :

I can't understand the meaning of the sentence.
Is hippopotamus here an mataphor? What is compared to hippopotamus ? The feet ? Or the body ? What does the writer mean to say using this metaphor ? Why does the writer compare it to hippopotamus ? Does the two things have something in common ?

It's just an exaggeration. Hippo is used here to emphasize that the stockings are in such a bad (i.e. ragged) condition that such a big animal can pass it without any difficulty. A very common expression. No need to make a fuss over it!儒勒·凡尔纳虽然在现代很有名，大家都知道他的《海底两万里》、《八十天环游地球》等作品，但当时却被认为是疯子，少有人赞赏他，因为：他的思想太超前了。历史的玩笑，对于活生生的、正在承受的人来说，却并不可笑。
By the way,mataphor应为metaphor