The basics are simple: right foot, left foot, repeat as desired. But as anyone who had to cut a swath through regatta spectators this past weekend can attest, it's a jungle out there. To aid the National Geographic Explorer in all of us, here are the four most prolific inhabitants of this particular ecosystem.

The old-timer: Easily identifiable by his practice of wearing striped crew socks all the way up to his knees. He looks as though he can breathe only with the assistance of an iron lung, but you are the one sucking wind as his spindly legs and reconstructed hip propel him past you faster than he can drive.

The marathoner: This species is distinguished by its remarkable endurance and unfortunate patterns of plumage, their lower coverings often well above the acceptable limit. These creatures will often anticipate your route, staying just in front of you, offering a generous view of their poorly concealed hindquarters. Actually maintaining this view for more than a quarter-mile results in blindness and eventually death.

The walkers: Usually seen traveling in groups, these organisms like to occupy all of the space alloted by Cambridge's narrow paths. Passing a gaggle of these could be disastrous: an ill-timed swing of a one-pound weight or walkman could send you reeling either into the muddy banks of the Charles or in the middle of oncoming traffic. Instead, charge through the middle of the pack, paying no attention to their resulting bellows.

The cyclist: Their hard exoskeletons make them a force to be reckoned with. As a general rule, they are always going the opposite way that you are, and will not alter their path even in the face of imminent collision. Try to gain eye contact with the beast, then hit the deck. Take care not to antagonize the cyclist--too-tight shorts and poorly-designed bicycle seats make them particularly ornery.