seventeenth

here is explained about an animal "Z".tell me what is X,Y and Z

"In the river X a ---- is found resembling those which are usually found on fig-trees. Its average length is seven cubits, though some are longer, others shorter. It is so thick that a child ten years old could hardly put his arms round it. It has two teeth, one in the upper and one in the lower jaw. Everything it seizes with these teeth it devours. By day it remains in the mud of the river, but at night it comes out, seizes whatever it comes across, whether ox or camel, drags it into the river, and devours it all except the intestines. It is caught with a large hook baited with a lamb or kid attached by iron chains. After it has been caught, it is hung up for thirty days with vessels placed underneath, into which as much oil from the body drips as would fill ten Attic kotylae. At the end of the thirty days, the worm is thrown away, the vessels of oil are sealed arid taken as a present to the king of Y, who alone is allowed to use it. This oil sets everything alight--wood or animals--over which it is poured, and the flame can only be extinguished by throwing a quantity of thick mud on it."