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; _c^1_, second or middle
segment; _c^2_, 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; _a^1_, supposed second or middle
segment; _a^2_, 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; _c^1_, second segment;
_c^2_, 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; _b^1_, thickened portion of membrane, perhaps
answering to the second segment of the mandibles; _b^2_,
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 _c^2_ 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; _d^2_, 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.
_e^2 e^2_, thoracic segment, supporting the second pair of
cirri.
_f_, _f^3_, 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.