Brooklyn's They Might Be Giants are a Type A band: total overachievers. Apollo 18, the band's fourth album, has 18 full-fledged songs on it, as well as countless unexplored byways of tunes- that-might-have-been. The pop duo John Linnell on accordion, John Flansburgh on guitar is attracted to comic-book-inspired topics, which they subsequently inject into their music with dorky charm. On the indelibly catchy song ''Mammal,'' for example, in the midst of jauntily reciting a list of cute animals, singer Flansburgh suddenly informs listeners who may have forgotten Bio 101 that a mammal is a creature ''with red blood cells, lacking nuclei.'' Sometimes the Giants get bogged down in their own cleverness on ''Fingertips,'' for example, they whip through 22 sound bites without ever settling on a melody. For the most part, however, the band's mad-scientist demeanor and reliance on good old-fashioned tunefulness makes for enjoyable, if terminally lightweight, listening. B+
