The Foregut ofGecarcinus lateralisas an Organ of Salt and Water Balance

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Abstract
The permeability of the foregut of the land crab, Gecarcinus lateralis, to tritiated water (THO), Na22, and Cl36 was studied in vitro during the intermolt period and after ecdysis. In crabs with eyestalks, the foregut is permeable to water and ions in the direction hemolymph-to-lumen and lumen-to-hemolymph, both during the intermolt period and after ecdysis. However, the foregut of animals without eyestalks is impermeable after ecdysis. The movement of THO always follows the movement of Na22 across the wall of the foregut, while the movement of Cl36 may or may not be correlated with the movement of Na22 and THO. Comparison of the ratio of water to ions in the hemolymph with the ratio calculated from radioisotope flux indicates that Na+ and water are probably moving isosmotically, although not necessarily accompanied by Cl− When an extract of the thoracic ganglionic mass of G. lateralis is added to the “hemolymph side” of the foregut in vitro, there is immediately a large increase in permeability to water and salts. This occurs in the foregut of crabs with eyestalks during intermolt and also in eyestalkless crabs after ecdysis. Thus, not only is the foregut of Gecarcinus lateralis permeable to water and salts in both directions, but also the extent of its permeability is under neuroendocrine control. As a consequence, the animal may be able to move water and salts into the foregut or out of it into the hemolymph as needed. This may be an important adaptation for a terrestrial crab that must conserve water, especially at the critical time of ecdysis.

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