14 DISCOVERY REPORTS 



which prevents the openings of the digestive glands from being occluded by large particles is uncertain, 

 but the following is a possible interpretation of the structures found. The long projections on the 

 lateral and ventral pads form a triple barrier across the lumen. This barrier is augmented by the long 

 setae on the lateral pads. As a large mass of food is passed back it comes up against the barrier and 

 depresses the projections so that a bridge is formed which guides the particles across the pyloric 

 chamber into the intestine. This movement causes the lobes from which these projections rise to be 

 bent backwards and downwards to fill a large part of the ventral chamber, occlude the openings of 

 the digestive glands and at the same time press digestive secretion from the chamber out on to the 

 food as it enters the intestine. Elasticity for this movement is provided by the large blood sinuses 

 within the lateral pads and below the ventral one just anterior to the point where they become free. 

 When the food has passed, the projections spring back to the vertical position assisted by the powerful 

 muscles in the lateral lobes of the process. The pyloric chamber is thus opened once more and ready 

 to be refilled with secretion from the digestive glands. 



The structure of the digestive glands is such that they are probably almost entirely secretory, while 

 a little digestion and absorption of the small amount of finely divided material which enters them 

 may also take place. 



The dorsal caeca are entirely secretory as in Nebalio. 



The structure of the intestinal wall suggests that besides absorption there is additional secretion 

 of digestive enzymes especially in the anterior region. 



MODE OF FUNCTIONING OF THE GUT OF NEBALIOPSIS TYPICA 



The mode of functioning of the various parts of the gut of Nebaliopsis cannot be described with 

 certainty as yet, for in specimen E hardly any particles were present and in specimen F2 the digestive 

 sac was full of an almost homogeneous mass resembling coagulated yolk, but two alternative mechanisms 

 are here suggested, the second being the more probable. 



I. Fine particles filtered out of the water by the maxilla and first trunk limb (Cannon, 193 1) may 

 be sucked into the stomach by the action of the lateral dilators and the circular muscles. There can 

 be no preliminary grinding owing to the structure of the mouthparts, but once within the gut any 

 large pieces may be ground between the dorsal ridge {d.r. Fig. 4) and the lateral thickenings {l.th.) 

 and also between the side walls which approach each other very closely and are heavily chitinized 

 and grooved (Fig. 5 C, D). There are no setae to hinder direct passage of food into the digestive sac, 

 therefore it cannot remain long in the fore-gut. Digestive secretion is poured on to it as it passes 

 the openings of the anterior digestive diverticula {d.gl. Fig. 7 A). These openings are large and 

 unprotected and particles could easily enter them, but the structure of the glands does not suggest 

 that any absorption takes place within them. 



There is no possibility of any food passing straight from the fore-gut to the intestine as it does 

 in Nebalia and Nebaliella. Everything must enter the sac where both digestion and absorption 

 probably take place. 



It is difficult to visualize how the sac does not become clogged with indigestible matter, as there 

 is no apparent means of circulating the material in it. A possible explanation is that a deep pelagic 

 filter feeder will obtain very little particulate inorganic matter such as is so abundant in and near 

 the surface of mud so that digestion will be almost complete. Filterable particles are scarce in this 

 zone, and the blind diverticulum permits the retention of all material until it is thoroughly digested 

 thus preventing waste. 



This suggested mechanism agrees with Cannon's belief that Nebaliopsis is 'entirely a filter feeder'. 

 His conclusions were reached from a study of the mouthparts alone, particularly important being the 

 facts that 'the whole mouth armature is extremely soft and unsuited for dealing with large food 



