THE GUT OF NEBALIACEA 15 



particles', and that 'in addition there is a compUcated structure which, in my opinion, must be a 

 fiher'. The internal organization, however, and other considerations, make probable the following 

 alternative mechanism, in which the first of these facts plays an important part. 



II. Nebaliopsis is probably an egg sucker. As far as information is at present available eggs of 

 various types have been found in small numbers in all the hauls in which Nebaliopsis has been collected. 

 The mechanism by which it sucks the yolk from these eggs is probably as follows. 



As Cannon reports, the mandibular palps are long and armed at the tips with stout claw-like setae, 

 which grip the slippery surface of the egg. The eddy currents round the mouth caused by the move- 

 ments of the trunk limbs and mouthparts also help to keep the egg pressed against the mouth. The 

 molar processes of the mandibles are soft and useless for biting ; thus the egg is not punctured outside 

 the mouth, where there would be great risk of the contents being washed away by the water currents 

 in that region. 



While being held close to the mouth one side of the soft egg is sucked into the oesophagus by the 

 pumping action of the muscles on its walls. It is then gripped tightly by the plated surfaces of the 

 lateral walls, while the biting action of the dorsal ridge against the dorso-lateral thickenings of the 

 chitin makes a hole in the egg membranes. The liquid yolk is then pumped through this puncture into 

 the digestive sac, digestive secretion being poured on to it as it passes the openings of the anterior 

 glandular caeca. The great thickness of the muscle sheath of the fore-gut may be associated with this 

 strong pumping action. The steadying action of the mandibular palps and the eddy current round 

 the mouth are most important during this process. The empty egg case would then be thrown away. 



The capacity of the digestive sac is sufficient to accommodate the contents of several average-sized 

 fish eggs. As suitable eggs are likely to be found in groups near where they have been spawned, and 

 only at certain times of year, a meal is available only at infrequent intervals. Much food is thus taken 

 at one time and is stored in the immense digestive sac where it is assimilated slowly as required. 



There is hardly any solid waste in this method of feeding, which agrees well with the observed 

 structure of the extremely narrow intestine and the lack of through current or strong muscles by which 

 solid waste could be evacuated from the blind digestive sac. It also agrees with the fact that no 

 ' structure ' was found in the solidified mass in the sac, as would be expected if particulate matter were 

 collected indiscriminately by a filter mechanism or indeed if Nebaliopsis fed on anything but liquid 

 or semi-liquid food. There is nothing in the structure of the mouthparts or gut to suggest that it is 

 a blood sucker, and the appearance of the food undoubtedly suggests coagulated yolk. 



In the depths from which Nebaliopsis has been collected there can be very little finely divided 

 material for a filter feeder — only the slow rain of dead plankton from the surface layers. An animal 

 of the size of Nebaliopsis will require a considerable amount of food. The second theory would supply 

 this better than the first. As has been shown, the structure of the gut and especially the presence of 

 the large digestive sac also indicate that an occasional large meal is taken. It is possible that the 

 animal depends chiefly on sucking eggs, but has a filter mechanism which provides a small additional 

 supply of food, alone insufficient, but valuable when prey is scarce. 



Without intermediate forms it is impossible to tell how this complex and highly specialized mechanism 

 originated. It is undoubtedly, however, well adapted to the environment in which the species now lives. 



CONCLUSIONS 

 The structure of the gut diff'ers considerably in the different members of the Nebaliacea, and many 

 of the changes may be correlated with the feeding habits. 



The greatest similarities are found in the musculature. It is obvious that for the efficient working 

 of a complicated chitinized apparatus simple peristalsis of circular muscles is insufficient. Opposing 

 dilators are necessary. The oesophagus of Crustacea almost invariably has lateral dilator muscles 



