Sentence:  The cercariae contain massive glands, formerly considered to be penetration glands, but now believed to produce the slime responsible for the formation of these balls. It is likely that the slime produced by the snail itself may contribute, at least partly, to the formation of a <WSD>slimeball</WSD>. 
Meaning of the word marked with <WSD> tags:  A mucus-coated lump containing the (parasitic larvae) of a liver fluke (of the phylum Platyhelminthes).