ABSTRACT
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.
One individual P. variant was observed to swallow water through the anus when the intestine was empty, behaving in a way which I have not otherwise seen in prawns. There were periodic bursts of rectal swallowing of water; 13 successive bursts were observed (at 23°C.), each burst having an average of 27 gulps with a rate of about 1 gulp per second, followed by an average pause of 28 sec. Intestinal antiperistalsis accompanied each burst of rectal pumping, otherwise there was none. At the end of each burst of rectal swallowing, a sausage of water descended the intestine as if it had been a piece of faeces, then one or two more gulps followed, after which came the pause. This was a peculiar case of rhythmic pseudo-defaecation. It corresponds to the normal occurrence in small Crustacea which show continuous anal swallowing of water proceeding unabated when the gut is empty, with momentary interruptions for a pseudo-defaecation of water. This was described above in Leptodora. and in Sida which had been made to swallow Chlorella through the anus.
Jančařík (1949) has observed that Daphnia and Moina occasionally suck air into the intestine through the rectum and that the bubble is resorbed through the gut wall. He considers this to be evidence in support of intestinal respiration. But if a bubble of air enters the gut it must inevitably decrease in size as its oxygen diffuses into the tissues, where, owing to tissue respiration, the oxygen pressure is lower than in the bubble. Moreover, the swallowing of air must be very exceptional as it has never been observed in this laboratory where living Daphnia has been studied continuously through the last decade.
In Leptodora the narrow anterior part of the midgut is known as the ‘oesophagus’ and the posterior wide part as the ‘stomach’. In other Crustacea the foregut, here called the ‘gullet’, is often called the ‘oesophagus’.
The figures in brackets denote the mean number of swallowing movements per minute. The temperatures were between 17 and 23°C., but for simplicity are not given.
In the marine cladocerans Podon and Evadne, although I failed to see either oral or anal water intake, antiperistaltic contractions of the midgut wall were continuous. There was also a continuous, more rapid, rhythmic contraction by the short, wide anterior gut caeca. In Evadne nordmanni at 22°C. the gut wave frequency was once in 40 sec., that of the caeca once in 5 sec. The effect of the caecal contractions would be to keep up a rhythmic pressure on the water in the gut lumen, thus distending the gut walls, and so, presumably, enabling their muscles to contract effectively. It would be interesting to know if the caeca have a more powerful musculature.
One wonders why these movements are antiperistaltic, not peristaltic. In the Decapods antiperistalsis of the hindgut intestine clearly serves to move the water, which has been pumped in by the rectum, forwards along the intestine, and past the faeces which might otherwise act as a plug causing the swallowed water merely to swell out the hind portion of the intestine. As it is, the water is moved forward and stretches the gut until defaecation takes place. But in the cladoceran midgut intestine a backward peristaltic movement would presumably have served as well as a forward antiperistaltic movement to churn up the food: perhaps an antiperistaltic movement is inherent in crustacean organization, as seen in the rhythm of movement of phyllopod and other limbs.
Miss Barbara M. Gilchrist has observed that in Branchipus stagnalis the contractions of the lobed anterior intestinal caeca are synchronized with another rhythmic movement : they contract each time that the gullet makes a peristaltic swallowing movement, thus driving the gulp of water or food along the intestine.
In Daphnia defaecation is very rapid; when it occurs, intestinal antiperistalsis and rhythmic contractions of the caeca stop momentarily.
