azo DISCOVERY REPORTS 



When a Nebalia burrows in the mud it depresses its rostrum and partially closes its 

 carapace so that the front end of the carapace will not become swamped with mud and 

 the feeding mechanism choked. I assume that Nebaliella behaves in the same way. 



Now when the rostrum of Nebaliella is depressed the eyes must automatically rotate 

 inwards. In fact, as will be seen from Fig. 7, the rostrum cannot be depressed without 

 the eyes first of all shifting in some way, since the upper limit of the front margin of the 

 eye rests against and appears to support the under surface of the flange of the rostrum. 

 The flange, however, is not flat but concave ventrally (Fig. 7 B). Its ventral surface thus 

 faces downwards and inwards. Pressure of the flange on the upper margin of the eye must 

 therefore force the latter inwards and, since the eye is lodged by its heel against the base 

 of the antennule, this pressure can only have one result, that of rotating the eyes inwards. 



As the depression of the rostrum continues the eyes will swing inwards until the 

 upper parts of their anterior margins come to lie alongside the median keel of the 

 rostrum. The latter will then be gripped between the eyes and, together with them, will 

 form a lid tending to close the anterior opening between the valves of the carapace. This lid 

 cannot be further depressed on to the carapace edges unless the eyes can move backwards. 



On burrowing, as I have stated above, I assume that Nebaliella partially closes its 

 carapace. In order to do this, it must withdraw its antennules or, at least, their upper 

 parts, backwards into the carapace. This will allow the heel of the eye to move slightly 

 backwards, and this, in turn, will allow the rostrum to fold down to its maximum 

 extent until its flanges rest oh the anterior edges of the carapace, the tips of the eyes 

 passing just inside the carapace. 



Thus when a Nebaliella is burrowing the rostrum and eyes together form a lid closing 

 the upper and greater part of the anterior opening of the carapace. It is possible 

 that the distal parts of the antennules will project as sensory feelers through the lower 

 part of this opening just ventral to the tips of the eyes. But for the greater part this gap 

 will be closed by the massive third and fourth joints of the antennae. 



From Fig. 7 A it can be seen that the first and second joints of the antennae are of 

 such a length that the powerful spines on the third and fourth joints project just beyond 

 the edges of the carapace (Fig. 6), and I suggest that these are the structures which 

 Nebaliella uses to push itself through the mud. They do not occur in any other Nebalia- 

 cean and it is possible that they have some other function, but it is because of their 

 strength and arrangement, and from the fact that, in the Discovery specimen they were 

 completely covered with mud, that I suggest that they form the chief burrowing organ. 

 They are situated along the inner lower margins of the two joints and, on the fourth 

 joint along the inner edge of a ridge under which fits closely the edge of the carapace. 

 The latter can thus be almost closed ventrally and yet allow these antennal claws to project. 



The separation of the third and fourth joints supports my suggestion, as, separated, 

 they have much greater power for gripping the mud than if they were fused. In the 

 latter case a movement of the whole antenna would be necessary for them to dig into 

 the mud, while as they are, they can do this by rotating about their common joint and 

 about the joint between the third and second segments. 



