Invertebrates of the upper chamber,Gatún Locks,Panama Canal,with emphasis on Trochospongilla leidii (Porifera)

Observations were made on the horizontal distribution of certain invertebrates on the walls and floor of the upper chamber of the Gatún Lock system of the Panama Canal. The hydroid Cordylophora caspia (Pallas) and the gastrophod Neritina usnea Röding extended the full length of the chamber; the oligochaete Marionina sp. was confined to the upper half of the chamber (adjacent to Gatún Lake), and the amphipod Gitanopsis tortugae? Shoemaker, the isopod Munna sp. and the bivalve Mytilopsis sallei (Recluz) to the lower half; the sponge Trochospongilla leidii (Bowerbank) was distributed from the upper end to near the lower end of the chamber. A peculiar wedge-shaped distribution of T. leidii is thought to be a response to the influence of slight increases in salinity. Morphological observations on T. leidii are presented.