12 DISCOVERY REPORTS 



but he does not mention these points as differences. Therefore either his description is inadequate 

 or his diagram incorrect. In the absence of further material the answer to this question cannot be given. 



MODE OF FUNCTIONING OF THE GUT OF NEBALIA BIPES 



The mode of functioning of the gastric mill of Nebalia bipes may be deduced from evidence furnished 

 by the structure of its parts, the distribution of particles within the gut, and also from observation of 

 living animals. 



Specimens kept in shallow water in a jar, on the bottom of which was mud from their natural 

 habitat, were observed undisturbed. They occasionally swam about, but usually lay on the surface 

 of the mud (often in the shadow of large pieces of seaweed or stones), where it could be seen that 

 the thoracic limbs seldom ceased their regular rhythmic motion even when the animal as a whole was 

 stationar}^ They appeared to burrow only when disturbed. 



When isolated in small dishes and placed under the microscope they swam rapidly but at times lay 

 quiescent and could then be studied. The thoracic limbs continued to beat unless the specimens 

 were kept long in these conditions when they frequently became completely inactive for considerable 

 periods though often reviving later. When all the movement of the limbs ceased in this manner the 

 rate of heart beat slowed down and large particles were seen floating in the blood stream. This effect 

 has not been studied in detail but it is probably caused by the unnatural conditions in the dishes, as 

 no such long pauses in the motion of the thoracic limbs were noted when watching the animals 

 in the jar. 



The currents produced by the movements of the thoracic limbs bring particles to the filter apparatus 

 (Cannon, 1927). There is thus a continuous supply of food depending only on the concentration of 

 suspended matter in the water. If excessive amounts are collected the particles are gathered into balls 

 and shot out ventrally in the anterior region of the carapace. This mechanism probably helps to 

 prevent the filter apparatus from being choked with mud when the animal is burrowing and also 

 indicates that the movement of the thoracic limbs serves another purpose besides feeding. It is 

 possible that the continuous current of fresh water is necessary for respiration and must be maintained 

 whether it bears many or few particles. Thus normally there is a constant stream of filtered material 

 being passed to the mouth. Large particles have a preliminary grinding by the maxillary endites 

 (see Cannon, 1927, for details), and are also ground between the mandibles. 



Rows of setae on the lips prevent pieces from falling off into the grooves on either side of the 

 mandibles and direct them into the oesophagus. Strong contraction of the circular muscles keeps the 

 passage from the oesophagus to the stomach closed most of the time, but periodically these muscles 

 are relaxed, and simultaneously the lateral dilators work actively causing a 'puff' of particles to pass 

 into the stomach and swiftly back into the pyloric region and the intestine. Setae on the walls of the 

 oesophagus point dorsally and prevent backflow. All the gut muscles move violently during this 

 operation. 



The anterior median projection and the lateral plate setae {a.tn.p. and l.p.) help to direct the current 

 round the angle between the oesophagus and the stomach so that large amounts of material do not 

 pass dorsally and choke the grinding tubes. The setae are, however, not close enough to form a strict 

 filter, and some particles pass up into the grinding tubes and are ground between the setae (^.^.1) 

 and the vertical ridges {v.r.) and spines {g.s.o), which are rubbed across one another by a complex 

 circular and see-saw motion of the dorsal ridge, easily seen in living specimens and probably caused 

 by alternating contraction of the dorsal dilators combined with peristalsis of the circular muscles. 

 Only liquid was found in the dorsal channels (d.ch.), which are open posteriorly, and it is possible 

 that a secretion from the dorsal caeca may flow forwards in them and be poured upon the food as it 

 is being ground up in a manner analogous to Yonge's suggestion for Nephrops (Yonge, 1924). 



