Mesklin

The Hunter and his quarry raced through space—when suddenly both ships crashed head on into one of the oceans of earth!

Both the Hunter and his quarry were symbiotes—creatures who had to live within the body of another organism.

When the Hunter emerged from his wreck of a ship, he soon realized that a man of earth would make a suitable host. It became obvious, too, that his quarry had already selected his host—but which human being.

Then the Hunter found an excellent host in the person of young and intelligent Robert Kinnaird. He made his presence known and, in return for some clever suggestions and help with his search, he helped Robert with his problems. There follows an exciting, excellently wrought, and—you will agree—absolutely unique chase and deduction story. Tense, dramatic, and humorous, too, it's a top-drawer tale of scientific ingenuity. — From the dust jacket

Originally published in Astounding Science Fiction. Also published as From Outer Space, followed by a sequel:
Through the Eye of a Needle