_General Appearance._--Shape, flask-like, compressed (Pl. V, fig. 9, magnified 36 times), with a short neck: the outline is usually symmetrical, but sometimes is a little distorted on the under side. The creature is imbedded more than half its length or depth in the transparent, spine-bearing chitine border of the scutum of the hermaphrodite. Its length, or longer axis, varies from 10 to 11/400ths; its breadth, or transverse axis, is 6 to 7/400ths; and its thickness, for it is much flattened, is only 4/400ths of an inch. On the summit, there is a fimbriated orifice (_a_), the size of which can rarely be made out quite distinctly, owing to the extreme thinness of the membranous edges. A little way beneath the orifice, there are four little blunt, bristly points (_b_), generally rather more than the 1/1000th of an inch in length; they are rather variable in size, and seem to be of no functional importance; directly beneath them, there are four little calcareous beads (as may be known by their dissolving with effervescence in any acid, and breaking easily under the needle); these are the 3/2000ths of an inch in their larger external diameter; they are rather deeply imbedded in the outer integument, and taper a little downwards ending in a concave terminal point, into which a minute tubulus enters, like those passing into and through the valves of ordinary Cirripedia: along the axis of imbedment, they are often 4/2000ths of an inch in length. These calcareous beads or rudimental valves are seated in pairs, at the two ends of the flattened animal, so that when the animal is laid on one side, the upper bead in each pair exactly covers and hides the lower one. The outer integument is composed of chitine, as may be inferred from boiling caustic potash having no effect on it; the upper part is thicker than the imbedded portion and is wrinkled transversely; it is covered with minute spines 4/10,000ths of an inch in length, either single or in groups of two and three, (Pl. V, fig. 14.) This outer tunic is lined by corium, sometimes slightly mottled with dull purple; and this by delicate, longitudinal, striæ-less muscles, running from the base up to the under edge of the orifice; these longitudinal muscles are crossed, at least, in the upper part, by still finer transverse muscles.