NEBALIOPSIS 213 



from the mid-ventral space, although, as I have just stated, most of the water will 

 pass in through the lateral gaps. This will suck particles on to the long filter setae of the 

 limb. They will be swept off again into the mid-ventral space by the strip of short 

 setules of the limb in front (Fig. 5 A) just as, in the case of Nebalia, the fourth row 

 setae sweep particles off the first row. It is for this reason that I suggested that the 

 setules immediately behind the long first row setae may be homologous with the fourth 

 row setae of Nebalia. 



From Fig. 2 it will be seen that the combs of setae on the protopodites of the first 

 trunk limbs are largely exposed ventrally so that, if these setae and the maxillary setae 

 which they cover act as filters, there is this open ventral space from which water can 

 be drawn for filtering. In addition, if what I have argued above is correct, there is a 

 continuous stream from behind and more particularly a forward current in the mid- 

 ventral line which will be laden with food particles kept there by the setae of the trunk 

 limbs. 



The structure of the maxilla and first trunk limbs, in my opinion, indicate that they 

 must act as true filters. If this is so, it must be possible to demonstrate an actual filter 

 chamber and a mechanism by means of which water is caused to pass through that 

 chamber, and further, a means by which the filter itself can be cleaned and the residue 

 passed on to the mouth. 



The filter chamber is the inter-limb space between maxilla and first trunk limb. It is 

 a thin curved space close against and parallel to the body wall and roofed by the filter 

 comb of the maxilla (Fig. 5 A). The setae of this comb are armed with setules which 

 point downwards into the chamber so that, if water passes through this comb from the 

 chamber, any particles will be retained on the setules. The extent of these setules is 

 shown in Fig. 5 A where three enlarged drawings are shown of the setae near the 

 base, in the middle region and at the tip. These figures were drawn from an isolated 

 maxilla which was viewed from the median plane under a high-power binocular. The 

 arrangement of setules at the base of the setae was confirmed from the series of para- 

 sagittal sections. 



The floor of the filter chamber is formed by the filter comb of the first trunk limb. 

 Here again, the setae are armed with setules, and they point this time upwards, into the 

 chamber. As before, they would retain any particles carried by water if the latter passed 

 through them out of the filter chamber. 



There are no real walls to the filter chamber medianly, anteriorly or posteriorly, as, in 

 these directions, the tips of the setae of the first trunk limb touch the corresponding 

 setae of the maxilla ; but laterally, the chamber is bounded by the mats of setules which 

 project downwards from the maxilla and protrude into the similar setules which 

 project upwards from the first trunk limb (Fig. 5 A). This lateral wall must not be 

 looked on as a setal wall through which water could pass. It may be described as a 

 solid mass of setules and I believe it is just as effective as a solid wall would be. 



The manner by which water is drawn into this filter chamber is not very precise. 

 When the maxilla and first trunk limbs are lifted up from the body their inter-limb 



