In humans, the dartos is the tissue just behind the scrotum. As Ford is not human, there is no guessing what entangling his dartoid muscles would involve.
Early mini-computers in the 1970s used ticker tape read-outs. This is an oddly archaic computer for such a futuristic spaceship to have, but it works to great comic effect, in keeping with the general suspicion and distrust of computers Adams expresses thoughout this series of books.
The Horsehead Nebula is a dark nebula: an interstellar cloud that blocks out the light of stars. It is 1500 hundred light years away from Earth in the constellation Orion.
A split infinitive is a phrase where an adverb is inserted into the infinitive form of a verb, between "to" and the verb. While technically grammatically incorrect, it is commonly used for emphasis.
The most famous example of a split infinitive, and the one alluded to here, is "to boldly go", from the introductory narration to Star Trek:
Atlantis is a mythical island, which is said to have sunk beneath the waves of the Atlantic Ocean around 12,000 years ago.
A photon is the basic unit of light but, in this context, it's used as an expletive.
Eddie is singing 'You'll Never Walk Alone' from the 1945 Rogers and Hammerstein Musical 'Carousel'.