LABRAL GLANDS OF SIMOCEPHALUS 221 



characterized by the development of the proximal endite with 

 its comb-like row of setae \ These endites are placed almost 

 vertically with their setae pointing upwards into the food 

 groove. They diverge slightly from behind forwards and in 

 passing upwards towards the trunk they slope inwards. They 

 move in and out laterally. From the fact that they are nearest 

 together at their posterior end the outward movement sucks 

 in the water from before backwards. Since also they are not 

 placed vertically but are sHghtly further apart at their proximal 

 end than they are at the end of the comb of setae in the food 

 groove, the outward movement, in all probability, causes 

 a small backwash in a forward direction in the food groove. 



Although the food current is produced mainly by the third 

 and fourth trunk-limbs the first also plays an important part. 

 The shape and arrangement of the first trunk-limbs can best 

 be seen from Text-lig. 1. Its setae form a curved shield over, 

 that is, ventral to, the second trunk-limb. In its normal move- 

 ment it synchronizes with the other trunk-limbs but is not in 

 the same phase. It commences its backward stroke just after 

 the other limbs begin to beat outwards. The outer part of the 

 limb moves in an arc of a circle with the tip of the labrum 

 as centre (Text-fig. 1) so that those setae which lie against 

 the side of the labrum scarcely move at all. The two limbs 

 together thus form a funnel-like entrance to the food passage 

 down the centre of which projects the labrum. The reason 

 for the retarded lateral movement of this pair of limbs is not 

 at all certain, but in all probability it is to secure a passage of 

 water over the branchiae. 



The second trunk-limbs are pecuhar in possessing a large and 

 specialized processus m axillaris or gnathobase. Their 

 exopodites or outer branches lie over, that is ventral to, the 

 succeeding limbs, and their function is probably merely to 

 assist by their oar-like movements in maintaining the food- 

 stream. The gnathobases point inwards and are beset with 

 setae which point inwards and almost meet in the middle line. 

 On. each gnathobase there are ten setae. The posterior seven 

 point backwards while the anterior three point forwards. They 



