The internal structure of these shells pre sents some peculiar features. •They all contain numerous tubes and cavities, regularly ar ranged, and forming a sort of diploe. The suture-holes mentioned above open each into a separate canal, chamber, or tube. Those which occur in rows on the walls of the cone lead to small chambers within the second series of compartments, running parallel with the general base, and separated from one another by delicately-formed partitions, each of which is deeply grooved on both sides. The par titions are placed at equal distances and their grooves are most regularly formed. 'The whole presents one of the most beautiful and delicate pieces of structure with which we are ac quainted in the whole rancre of extravascular skeletons. These are from thirteen to fifteen on either side of each partition. Fig.334 repre sents a perpendicular section of a few of these grooved par titions considerably magnified. Fig.335 represents a horizontal section of one of the six valves. The holes forming the sutures are at a. The grooved floor of one of the chambers of the piece is tween a, d, and c. d, c is the outer wall of the compartment of the cond series. a, b is a section of that part of the valve which appears outside as a ment of the first series. Its diploe is posed of tubes, running from the apex to the base, gradually enlarging below. Horizontal sections of those tubes shew them to be of an ovate form, tapering inwardly (fig. 336). They are placed nearer the outer wall than the inner. The spaces intervening between the ing sides of the tubes are marked with lines of growth, ing a gradual filling up of th.e tubes from within outwards ; and also the previeus istence of furrows or grooves on the' surfaces of the partitions between the tubes. These grooves are vcry strongly marked iii some cies_ as in Bainnus Spinosus (fig. 337), where the tubes are large, and the walls comparatively thin. In all they run in straight diverging lines from the apices of the compartments to their bases. There they open close to the margin of the general base. In most species, however, their orifices are, in part, filled up by an extension of the base (a, fig. 338). In some small species, the tubes of which are wider than those of larger ones, there is hardly any opening discoverable externally, or at most a very narrow fissure just around the margin. Very near their terminations on the gin, these tubes of the diploe are joined by the very short canals which proceed from the inner circumference of the base (6, fig. 338), and it is at their junction that the grooves in the walls of the partitions are most obvious. These two sets of tubes communicate freely all around the margin with the diploe of the base. All the Balanids—with the exception of the Coronules—have calcareous bases. The struc ture of the base differs from that of the walls in being composed internally of large oval cells irregularly arranged. These cells seem to communicate freely with one another and with the tubes of the valves. The Coronules have no base : their soft parts are in immediate contact with the integuments of the living animals in which they are generally imbedded.
The form and arrangement of the opercule vary. There are generally four triangular valves, two larger than the others, all deeply grooved on their upper surfaces by the lines of growth. These valves cover more or less completely the soft parts' beneath, to which they are attached, so as to be very moveable one upon the other, and to admit of the pas sage of the feet through the slit that exists be tween the two pairs. In some of the coronules, the greater part of the opercule is soft. Coro nula diadema has two small shelly plates in its opercule.
Keeping in view the complex but beautiful structure just described, it is not difficult to determine how the whole shell increases in size. It is obvious that the parietal compart ments of the first series are enlarged by addi tions to their basilar edges and internal surface, and that thus the whole cone is lengthened, and consequently widened at its base ; but, in all the species, it is also widened above ; and, as the summits of the first series of com partments are, evidently, not at all, or, at most, very slightly, abraded by the friction of the opercule, it is certain that the apices of these compartments—originally very closely approx imated—must be moved outwards and sepa rated from one another by the g,radual increase in breadth of the intervening wedge-like com partments of the second series. This process
)mplics the insertion of soft parts endowed with vascular action between the valves so as to admit of lateral additions being made to the second set of compartments. There can be no question that these soft parts (foliated processes of the mantle) pass into the sutures along their whole length, and .deposit .the shelly matter on the edges of the partitions forming the chambered structure of the se cond series of compartments ; each valve, with the exception of the dorsal one, is thus added to in breadth ; and as the distance between the original valves is enlarged, and the whole shell lengthened, new chambers are formed below. Of course, as the cone is lengthened, its base is widened ; and this is effected by the excretion of shelly matter from such parts of the mantle as can easily pass through the numerous holes placed around the inner cir cumference of the base. The valves of the opercule are imbedded in the margins of the mantle between the epidermis and true skin, and are increased by marginal additions in the same way as the shells of molluscs.
The mode of growth of these shells engaged the attention of Cuvier, who concluded that an addition to the sides of the valves could take place only in an early ac,,e ; for it appeared to him that they are, in a more adva.nced stage, so firmly cemented together as not to admit of separation. In large species, however, we find that the valves are easily separated at the sutures, and that the calcareous matter along the sides of the sutures is loosely aggregated ; so that, to us, there seems to be no impro bability in the supposition that in the living animal the prolongations of the mantle pass between the terminations of the minute tubu lar processes of the second series of compart ments, and the corresponding. depressions in the edges of the first series already noticed. There is no indication, we think, of each of the valves being "detached from its neighbour only at certain times that it may receive addi tional calcareous matter along its sides," as Brugieres and Cuvier imagined. The process. of growth seems to be carried on in uniform progression until adult age. So puzzling did the problem of the mode of growth in these shells appear to Dufresne, that he concluded that, like crabs, the Balanid casts its old shell, and forms a new one, as it increases in size.* Cuvier remarked that, " while the mode of growth of the shells of the Mollusca resembles that of simple teeth, the organization and in crease of the shells of balanids may be com pared to that of certain compound teeth, par ticularly those of diodons and tetrodons." Tubicinella, a parasite of the Whale, differs much from the other balanids in the formation of its shell. The widest part of its six-valved cone is sUperior; .the whole surface is strongly ribbed, and marked with transverse lines of growth ; and it appears that the additions to the cone are made on the upper margin ; this margin is surrounded internally by a thick and fleshy production of the mantle, which is never altogether covered by the opercule. The base is open, and of little less diameter than the upper part, which led Dufresne to conclude that the animal does not form a shell until it be considerably advanced in growth. This seems to be very probable, as the base is im bedded deeply in the integument of the Whale, and descends lower the more it increases in size, so as to leave only the summit of the shell visible. The imbedded portion is gene rally deeply coloured by the tegumentary pig ment of the Whale. In coronula, which also inhabits the backs of Whales, but has the same general structure of shell as the majority of Balanids, the valves are deeply partitioned, and provided with toothed processes, fitted to fix the animal in its site.