THE GUT OF NEBALIACEA 13 



The particles from the grinding tubes are passed back and on to the long setae of the pyloric region. 

 The narrowness of the lumen of the gut in the posterior part of the cardiac section and the presence 

 of the ventral cardio-pyloric valve {v.v.), whose tip moves violently describing an ellipse, causes 

 particles which have been driven directly back without secondary grinding in the grinding tubes to 

 pass up on to these setae also. The latter filter off the larger pieces and bear them back beyond the 

 openings of the digestive glands and far down the intestine in the tubular extension of the dorsal 

 process. The smaller pieces fall through into the ventral pyloric chamber and pass into the digestive 

 glands {d.gl). Muscle bands on the walls of these glands probably cause pumping in and out of fluid 

 bearing small particles as in Nephrops (Yonge, 1924). Certainly in ink- fed specimens grains were 

 found far into these caeca indicating that particles from the stomach are passed into them. Particles 

 appearing like finely ground food were frequently found in them also. No ink grains or other particles 

 could be seen in either the dorsal or ventral caeca {d.ca. and v.ca.). This suggests that absorption as 

 well as digestion probably takes place in the digestive glands, while the dorsal and ventral caeca 

 secrete a digestive fluid only. 



The arrangements of the gastric mill of Nebalia are such that there is continuous action of the 

 secondary grinding apparatus which increases the number of particles small enough to pass into the 

 digestive glands, while at the same time the animal is able to deal casually with the large quantities 

 of potential food which are automatically available and whose amount depends only on the concen- 

 tration of particles in suspension in the water filtered and the proportion of inorganic to organic matter. 



MODE OF FUNCTIONING OF THE GUT OF 

 NEBALIELLA EXT REM A 



Nebaliella is a mud-living form. The eyes and rostrum have been shown by Cannon (1931) to be a 

 mechanism whereby mud is prevented from entering the space within the carapace and choking the 

 filter apparatus as the animal burrows. Variations in the completeness of closure of this apparatus 

 control the current entering the filter chamber. Particles found amongst the mouthparts and also 

 within the gut include large pieces of diatom skeletons, radiolaria, and many unidentifiable broken 

 pieces showing that the animal is an indiscriminate mud feeder and also that it can deal with relatively 

 coarse filtered food. That these particles are present far down the intestine indicates that there is no 

 very efficient grinding of the food. It is probable that as in Nebalia much material is passed through 

 rapidly and a little is more carefully treated. 



That particles passed on to the mandibles receive only slight grinding before entering the oesophagus 

 is shown by the state of the food within the gut. The sheath of circular muscles probably functions 

 in the same way as in Nebalia and releases particles spasmodically. 



The angle between the oesophagus and the stomach is more obtuse than in Nebalia, the anterior 

 median projection is much reduced and the lateral plate setae are absent, but clogging of the grinding 

 setae {g-s.-^) is prevented by an entirely different mechanism. In the anterior region the edges of the 

 horizontal shelves of the side walls almost touch the small ridges {d.r.') on the dorsal ridge so that 

 the channels containing the setae (^.^-i) are nearly closed and only relatively small particles can enter 

 them. These particles are ground between the setae and the shelf and when fine enough pass between 

 the former and are found as a ' felty ' layer on the dorsal side of them. This arrangement and the 

 general reduction in the numbers of setae are almost certainly correlated with the coarse texture of 

 the food against the passage of which fine setae would have no effect. Such setae would soon be 

 broken or worn away. The spines (^.^.2) may have some guiding effect on the current, but as they are 

 so short they are probably only a relic of their homologues in Nebalia. 



There is no filter mechanism in the pyloric region. The openings of the digestive glands are small, 

 and only most minute particles were found within them. In the absence of more material the mechanism 



