Apparatus for introducing webs of exposed photographic paper into a developing machine has a feed table with one or more channels communicating with a chamber below the table. The effective length of each channel can be varied by a plate-like selector which is slidable on the table and has several apertures each movable into register with the respective channel to define with the latter a passage for the leader of a web which is stored in a cassette on the table and is about to be introduced into the chamber for attachment to a threading strip which advances the leader through the tanks of the developing machine. The selectors can be locked in selected positions by discrete detent devices and have indicia to indicate their positions with respect to the feed table. A partition is insertable between two neighboring selectors to close the median part of a relatively long channel whereby the outer parts of such longer channel constitute two shorter channels whose effective length can be changed by appropriate shifting of the corresponding selectors.