Cirrhopoda / [John Coldstream].

  • John Coldstream
Date:
[1837?]
Catalogue details

Licence: Public Domain Mark

Credit: Cirrhopoda / [John Coldstream]. Source: Wellcome Collection.

    5/16 (page 5)
    CIRRHOPODA. of the first series, as they appear externally, has attached to its dorsal margin one of the second series, and that the union between these two is exceedingly intimate, in fact that they form one piece, notwithstanding their apparent division externally. Two of the second series of compartments are attached to the anterior valve, while the dorsal valve has none. The anteal margins of the lateral valves and both margins of the dorsal valve are marked by transverse depressions corresponding to the numerous partitions of the chambered com¬ partments which are fitted into them; and, externally, each has a projecting margin. To the upper part of the inner surface of each valve there is attached a laminated process, form¬ ing part of a circle of calcareous plates which gives support to some parts of the mantle. 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 range of extravascular skeletons. These are from thirteen to fifteen on either side of each partition. Fig. 3 repre¬ sents a perpendicular section Fig. 3. of a few of these grooved par¬ titions considerably magnified. Fig. 4 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 be- Fig. 4. tween «, d, and c. d, c is the outer e wall of the compartment of the se¬ cond series, a, b is a section of that part of the valve which appears outside as a compart¬ ment of the first series. Its diploe is com¬ 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 (jig. 5). They are placed nearer Fig. 5. the outer wall than the inner. The spaces intervening between the taper¬ ing sides of the tubes are marked with lines of growth, shew¬ ing a gradual filling up of the tubes from within outwards; and also the previous ex¬ istence of furrows or grooves on the surfaces of the partitions between the tubes. These grooves are very strongly marked in some spe¬ cies, as in Balanus Spinosus (Jig. 6), where the tubes are large, and Fig. 6. 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, Jig. 7). 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 mar¬ gin, these tubes of the diploe are joined by the very short canals which proceed from the inner circumference of the base (b, Jig. 7), 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 gradual increase in breadth of the intervening wedge-like com¬ partments of the second series. This process Fig. 7.