PLATE 26.
STRUCTURE OF THE MOUTH AND THORAX.

N.B.—The same letters of reference are used for the parts of the mouth throughout this plate. All the organs here 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.
• b, maxilla.
• c, mandibles.
• d, palpus; d′ ditto, cut closely off.
• e, labrum, crest of.
• x x, first pair of cirri, cut off.
• 2, Supra-œsophageal cavity of the mouth of Balanus improvisus, torn open, with the palpi, mandibles, and maxillæ removed, exhibiting the inner face of the labrum, laid flat, and the inner faces of the outer maxillæ; scale as in fig. 1.
• a, outer maxillæ; 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.
• f, triangular thickened portion of the inner fold of the labrum; f′, rib or bar of membrane, thickened to support the surrounding very thin membrane of the supra-œsophageal cavity.
• g, opening of the œsophagus.
• h, forked bar of thickened membrane, for same purpose as f′.
• 3, Mouth of Coronula balænaris, divided vertically in a transverse line, with the outer and inner maxillæ and mandible on the left side removed, with the labrum, the other mandible and both palpi viewed on the inside, and with the œsophagus adherent in its proper position.
• c, Mandible, upper free segment; c¹, second or middle segment; c², supposed third or basal segment, forming the basal margin of the mouth, but not separated by any articulation from the labrum.
• d d, palpi; d′, 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, connecting the attached basal end of the palpus to the externo-lateral surface of the mandible.
• e′, central notch in the crest of the labrum; e′′ e′′, basal margin of the labrum, to which the membrane of the body is joined.

• f, thickened portion of membrane, part of the inner fold of the labrum, corresponding with f in fig. 2.
• g, orifice of the œsophagus, with the front part of the supra-œsophageal cavity cut away; g′, lower, expanded, or bell-shaped, end of the œsophagus, in its natural condition.
• i, k, concavities for the attachment of muscles running to the mandibles.
• m, cut off edge of membrane forming the side of the mouth, which cut edge can be followed up above the aperture d′, where the upper basal end of the palpus has been cut off, and so onwards by g, to the mandible on the opposite side of the mouth.
• 4, Mouth of Coronula balænaris, viewed from the outside, in front, with the inner maxilla, mandible, and palpus on the left side of the figure, and nearly the whole labrum, and the greater part of the palpus on the right side, all cut away. The maxilla, mandible, and basal portion of the palpus on the right hand, are pulled out of their proper positions; for the toothed edge of the mandible ought to have stood higher, and nearly in a line between and behind the outer maxillæ, and then the basal margin e′′ of the lateral portion of the labrum would be raised, and curling round, would stand nearly where the letter b′ is now placed.
• a a, outer maxilla, upper free lobe; a′′, lower and inner free lobe; a¹, supposed second or middle segment; a², supposed third or basal segment; a′′′, basal articulation of mouth, separating it from the ventral surface of the thorax.
• b, inner maxilla; b′, apodeme of do.
• c, mandible, upper free segment; c¹, second segment; c², third and basal segment, but not separated by an articulation from the labrum.
• d, palpus; d′′, membrane uniting the basal end of the palpus, where attached to the labrum, to the side of the mandible.
• e′′, basal margin of the labrum, on one side, displaced and pulled down.
• n, orifice of the olfactory pouch.
• 5, Balanus amaryllis: mandible, upper free segment, seen from the side that faces the labrum; p, an arched line, where the thickened membrane of the upper free part terminates, and is united by thin membrane to the near side of the palpus; q, ligamentous bands for the attachment of muscles.
• 6, Balanus amaryllis: palpus, on the same scale as the mandible, fig. 5, seen from the outside, so that the further or under face is the side which, towards the right hand, is united to the near face of the mandible, fig. 5; r, long bristles, springing from a protuberance near the extremity; these bristles, in many species, form a long single row, parallel to the basal margin; s, upper row of shorter bristles; t, row of very short bristles, arising from the further side and curling over the crest of the labrum.

• 7, Balanus amaryllis: maxilla on twice the scale of the mandible and palpus; b, upper free segment; b′, apodeme; b¹, thickened portion of membrane, perhaps answering to the second segment of the mandibles; b², thin membrane, extending down to the basal edge of the mouth, possibly answering to the basal segment of the mandibles; or perhaps the thickened membrane forming the protuberance c² in fig. 3, may be considered as the basal segment of the maxillæ.
• u, step-formed projection at lower angle of maxilla.
• 8, Thorax of Coronula diadema: outer membrane of the five posterior segments laid almost flat, and viewed externally as a transparent object.
• ² to ⁶, the second to the sixth cirrus, cut off a little above their basal articulations.
• a, basal curved end of the probosciformed penis.
• b, anus; b′, the membrane surrounding the anus, probably consisting of a rudiment of the abdomen.
• c, rectum, seen through on the under side.
• d, basal articulation of the sixth cirrus; d′, do. of the fifth cirrus; d², do. of the second cirrus.
• e e′, posterior thoracic segment, carrying the sixth pair of cirri; the dorso-lateral portion e is separated, in this one segment, from the corresponding portion e′ by a narrow slip of thinner membrane, which may be a part of the abdomen let in.
• e² e², thoracic segment, supporting the second pair of cirri.
• f, f³, folds or articulations between the adjoining segments.
• g, swollen, punctured portions of membrane, not differing homologically from the rest of the segments.
• h, posterior portion of the prosoma, or the specially enlarged part of the thorax; it carries the first pair of cirri; homologically h forms part of the second segment of the thorax of the archetype Crustacean.