PLATE 28.
CEMENTING APPARATUS.

• Fig. 1 a, Basal membrane, with the cementing apparatus, of Coronula balænaris; a small portion of the parietal membrane, p p p, which coats the folded shelly walls, is left adherent to the basal membrane.
• b, the circumferential slip (shaded more darkly than the rest to catch the eye), separating the basal from the parietal membrane.
• c′ c′, slips of basal membrane, formed at each period of growth, and overlapping each other.

• s s s s s s, the six sutures in the walls, separating the six compartments, of which—
• A is the basal margin of the rostrum,
• C C that of the lateral compartments,
• D D that of the carino-lateral compartments, and
• E that of the carina.
• r, r, rays or spokes of membrane, prolonged from the circumferential slip, and running under the trebly folded wall of each compartment, but here cut off; a similar ray should run under each line of suture (s). These rays, at their extremities, expand transversely; and the shape and length of the rays may be judged of from the basal outline of the folded walls given in Pl. 16, fig. 5.
• The two cement-ducts, proceeding from each cement-gland, debouch opposite the middle folds of the lateral (C C) and carino-lateral (D D) compartments. The layers of cement have been removed. N.B. There is one considerable error in this figure, the two main trunks, connecting the cement-glands, and meeting at the centre, have been represented as forming a straight line, but in fact they form a very open angle, as is correctly shown in fig. 1 c.
• 1 b, Diagram, representing a vertical section through a portion of the basal and parietal membranes, with the thickness of the membrane enormously exaggerated.
• z z z, layers of cement, which, if the section had been made in the line of the cement-ducts, would have been seen proceeding out of these ducts, as is represented at (t), where the section is supposed to have taken the above course.
• c′ c′, the slips of basal membrane.
• b, the circumferential slip; beneath this the coarsely dotted layer represents the cement, lately excreted, and before it has acquired its proper transparent structureless character, elsewhere represented by fine dots.
• p, membrane externally coating the walls of the shell.
• t, cement-ducts opening beneath the basal membrane.
• 1 c, The central portion of fig. 1 a, considerably magnified.
• c′ c′, slips of the basal membrane; but the shell, when these were formed, was so young that the walls had not acquired their folded structure; in the centre the prehensile antennæ of the pupa may be obscurely seen.
• f, the main cement-trunk, connecting the cement-glands.
• h, a cement-gland, from which two cement-ducts proceed.
• a a, cement-ducts (cut off), leading to opposite the middle fold of the carino-lateral compartment.
• b b, cement-ducts (cut off), leading to opposite the middle fold of the lateral compartment.
• a′ b′, a pair of cement-ducts, with their orifices opening on the under side of one of the slips of basal membrane (c′), at a point which once was opposite the middle of the carino-lateral and lateral compartments. The orifices of the other ducts, towards the centre, may be seen forming straight lines.

• 2, Cement-ducts and one cement-gland of Chelonobia patula, represented without the basal membrane, to which they adhere: f, f, main cement-trunk; g, enlarged portion; h, gland; a, b, the two ducts proceeding from this gland, and bifurcating several times before debouching on the under side of the basal membrane. Several other cement-ducts, proceeding from other glands, are represented just as they appeared under the microscope.
• 3, Cement-ducts and glands of Tubicinella trachealis, represented without the basal membrane to which they adhered: (f) (f), main cement-trunk, connecting the several glands; h, cement-gland; a, cement-duct, with a singular loop (a′) having two projections or rudimentary branches; b, spur or rudiment of a second cement-duct; c, third cement-duct.
• 4 a, Chain of cement-glands of Balanus tintinnabulum, with all the ducts removed, excepting those proceeding from the last formed gland, which latter correspond in age with the last-formed zone of the shelly basis; the whole of the basis having been removed by acid.
• f, main cement-trunk connecting the glands.
• gh, last-formed cement-gland.
• k, t, two cement-ducts, proceeding from a great common duct; one of these bifurcates at (t), and one branch joins at (t′) the corresponding branch from the corresponding gland.
• i i i, circumferential duct, into which the ducts k, k, t, t, t′ all enter.
• i′ i′, branches proceeding from the circumferential duct, which branch and sub-branch till they form a sheet (z z) of cement-tissue on the outside of the basis of the shell.
• 4 b, two cement-glands of Bal. tintinnabulum (this figure, to match with 4 a, ought to have stood upside down), taken from near the centre of the basis, greatly enlarged; (f f), main trunk; (g), enlarged portion of the trunk; (h), gland; k, t, two cement-ducts proceeding from a common point, one of them (t) bifurcates, and gives off a rudimentary branch, t′; m, a spur, or rudimentary duct. The gland, h, has been pushed on one side, it ought to lie over the enlarged portion (g). There is a considerable difference between these two glands and that (gh) represented in fig. 4 a; the neck of the gland in the latter being elongated into a great common duct, and the spur or rudimentary duct (m) being absent.