1. The composition and rate of production of fluid by the anterior and posterior midgut caeca (AMC and PMC, respectively) of the crab Cancer magisterwere studied in vivoand in vitro. Under both conditions these structures produced a small volume of fluid (2.2 - 3.4 microliters/h)which was, with the exception of sulfate ion, nearly isoionic with the serum.
2. Caecal fluid had relatively low levels of amylolytic and, less frequently, proteolytic activities.
3. The in vivorate of fluid production was not increased by feeding or by osmotic stress.
4. Ligation of the AMC did not impair secretion of a peritrophic membrane around the fecal strand formed after feeding.
5. It is concluded that the midgut caeca do not have a significane function in osmoregulation or formation of the perotrophic membrane and that they probably have, at best, a minor role on the digestive processes of C. magister.Further, it is suggested that the rate and direction of fluid transport by the midgut caeca may be under neurosecretory control.