STRUCTURE OF MOUTH. 79 



firmly united to the lateral corners of the labrum ; or indeed 

 the corners of the labrum may be almost said to be formed 

 by the soft, swollen bases of the palpi : the point of union, 

 when viewed from the outside, is seen to form a knob on 

 the shoulder of the labrum, beneath the level of its crest, 

 and at this knob (PL 26, fig. 3, close to d') several thick- 

 ened bands in the surrounding membrane unite. The free 

 portion of the palpus stands out transversely behind (i. e. 

 anteriorly to, in a homological sense) the labrum. I suspect 

 that the palpus possibly may consist of two segments, of 

 which the terminal one is free, and the lower one confluent 

 with the labrum. 



Before proceeding any further, I should observe that figs. 

 3 and 4, in PI. 26, represent the membranes of the mouth of 

 Coronula diadema, perfectly cleaned. In fig. 3, all the front 

 part of the mouth has been removed, the mandible on one 

 side, the labrum with the two palpi, and the oesophagus being 

 alone left, and these are viewed from the inner side; the front 

 part, however, of the supra-cesophageal cavity has been cut 

 away. In fig. 4, the labrum, with the oesophagus, has been 

 removed, whilst the two outer maxillae, the right-hand inner 

 maxilla and mandible (with the exterior and basal portions, 

 d } d" ', of one palpus) are seen from the outside ; but in order 

 that these parts should all be shown, the whole of the 

 right-hand side of the mouth has been spread out, for the 

 teeth of the mandible should have stood in a vertical line 

 between the two outer maxillae. In the mandibles, the free 

 upper part is separated, by a distinct articulation, from the 

 square piece of thickened membrane (fig. 3, c i) on which it 

 is supported ; and this latter is separated by a second articu- 

 lation from a portion of thickened membrane (c2), the basal 

 edge of which forms the third >and lowest articulation, sepa- 

 rating the mouth from the body. This basal, thickened 

 portion of membrane curls round and inwards, towards the 

 outer maxillse or front of the mouth, and its terminal points 

 sometimes even penetrate a little way within the muscles, 

 like apoclemes : it is not distinctly separated by any line or 

 suture from the membrane, which forms the whole broad 

 labrum ; so that I at first concluded that the labrum dipped 

 under the mandibles, and thus afforded a support on which 

 they were articulated ; but this appears so opposed to all 



