664 DESCRIPTION OF PLATE, 26 



PLATE 26. 



Structure or the Mouth, &c. 



N.B. — The same letters of reference are used for the parts of the mouth 

 throughout this plate. All theorganshere represented have been cleaned 

 by boiling in potash, and consist exclusively of the external membrane. 



Fig. 1, Mouth of Balanus perforatus, viewed from vertically above, 

 with the first pair of cirri (x x) in their proper position, 

 cut off close above their basal articulations. 



(a), outer maxilla ; (a') ditto, cut closely off. 



(6), maxilla. 



(<?), mandibles. 



(d), palpus ; (d') ditto, cut closely off. 



(e), labrum, crest of. 



(x x), first pair of cirri, cut off. 



2. Supra-cesophageal cavity of the mouth of Balanus improvisus, 



torn open, with the palpi, mandibles, and maxillae removed, 

 exhibiting the inner face of the labrum, laid flat, and the 

 inner faces of the outer maxillae ; scale as in fig. 1. 



(a), outer maxillae ; (a") inner and lower lobe of ditto. 



(d') } point of attachment to the torn-off palpus. 



(e), labrum, crest off; e', central notch, with graduated teeth 

 on both sides. 



(/), triangular thickened portion of the inner fold of the 

 labrum ; (/'), rib or bar of membrane, thickened to sup- 

 port the surrounding very thin membrane of the supra- 

 cesophageal cavity. 



(g), opening of the oesophagus. 



(A), forked bar of thickened membrane, for same purpose 



as (/')■ 



3, Mouth of Coronula balcenaris, divided vertically in a transverse 



line, with the outer and inner maxillae and mandible on 

 the left side removed, with the labrum, the other man- 

 dible and both palpi viewed on the inside, and with the 

 oesophagus adherent in its proper position. 



(c), Mandible, upper free segment ; (c 1 ), second or middle seg- 

 ment ; (c 2 ), supposed third or basal segment, forming the 

 basal margin of the mouth, but not separated by any arti- 

 culation from the labrum. 



(dd), palpi ; (d 1 ) aperture, through which the inside of this 

 palpus can be seen, caused by the mandible having been cut 

 off; the membrane on the under side of this aperture is 

 thickened, and affords the chief support to the palpus ; 

 (d"), thin membrane, apparently part of the palpus, con- 

 necting the attached basal end of the palpus to the externo- 

 lateral surface of the mandible. 



(V), central notch in the crest of the labrum ; (e") (e"), basal 

 margin of the labrum, to which the membrane of the body 

 is joined. 



