Anal and Oral Intake of Water by Crustacea

In small and transparent Crustacea water can be seen with a microscope to be pumped through the anus into the intestine. The pumping is done by antiperistaltic contractions of rectal muscles, the movements being repeated rhythmically at intervals of one to a few seconds. They may continue almost without interruption or may occur for short periods with intervals of rest. In some genera the entrance of water by successive gulps is quite clear; this is the case, for example, in Triops, Sida, Diaphanosoma, Leptodora, Mysis, Sphaeroma, Ligia, Corophium. In other genera, such as Artemia, Chirocephalus, Limnadia and Bythotrephes, it is not obvious that water enters, although waves of contraction are seen to pass inwards over the rectal musculature. In Daphnia the gulps cannot be seen when the antiperistalsis is slow, but they are clearly visible when it is rapid.