structure of the skin. During my investigations preparatory to the publication of that part of the first volume of the "Histological Catalogue" relating to the structure of the skeleton of the Asteroid

Zoophytes, knowing that spicula abounded in the skin of the Pennatula, I was desirous of ascertaining whether they were present in this animal, and after some search, was rewarded by finding them in the flesh at the lower part of the stem. Spicula of large size, however, are met with around the margin of the sheath into which the polypes retract themselves; they are very different from those in the flesh of the naked portion of the stem.

Having now described the general form and position of the skeleton in the family Pennatulidæ, I proceed to illustrate the minute structure of these parts. The transverse section of the bony axis of Pennatula grisea is of a cylindrical figure, and appears laminated to the naked eye, but when viewed with a power of 40 diameters, as shown by F, in Fig. 69, the outer margin or crust is found to be of a fibrous nature; the fibres taking a vertical direction, whilst the central portion, more dense than the rest, shows a cellular structure, the cells appearing to be filled with calcareous matter. The spicula in the skin from the base of the stalk, as shown at B, C, are of cylindrical figure, and somewhat twisted so as to produce the appearance of a central cavity; the ends are hemispherical, but their surface is smooth and free from tubercles, and they average

aboutth of an inch in length. The spicula from the pinnæ are fully half an inch long, they occur in bundles, some being pointed at both extremities, whilst others have one extremity expanded like the feather of an arrow, all being more or less striated internally; portions of two of these spicula showing striæ are represented by D, E; a similar striated appearance is exhibited by the spicula from the flesh at the base of the stalk, as shown at c. The spicula of the Renilla are precisely similar in shape to those of the Pennatula, but are of a rich purple colour, most of them average

th of an inch in length; all, as shown at H, are twisted in the same peculiar manner as those of the Pennatulæ before noticed.

Transverse sections of the calcareous axis of the Virgularia are of a circular figure, and exhibit a radiated structure, as shown by E, in Fig. 70, the true nature of the radii, however, is better seen in a vertical section, F, under a power of 130 diameters; they are then found to be produced by a series of minute sinuous canals, opening on the free surface, and passing in curved lines towards the centre of the stem, which is apparently more dense and opaque than the outer portion. The tissue between the tubes presents longitudinal striæ, and after the carbonate of lime has been removed by an acid, the organic residuum exhibits a more or less fibrous appearance. The transverse section of the bony axis of Pavonaria quadrangularis is of a square figure, with concave margins,

and, as represented by E, in Fig. 71, consists of a series of laminæ of fibro-calcareous matter arranged around a small central cross; the outer layer or crust is extremely hard and cannot easily be cut with a knife. The longitudinal section of the axis, as shown at F, exhibits a dark granular centre with vertical striæ on each side, and the organic basis of this structure appears to be of a fibrous nature. The spicula from the flesh in the neighbourhood of the polypes are of considerable length, and have both extremities pointed; they appear to be arranged in bundles, which project above the free surface in the form of bristles and alternate with the polypes, those represented by H being of the natural size. The spicula from the lower part of the stem are much more minute and of a flattened oval figure, most of them exhibiting traces of a striated structure, as shown at G; the long diameter of the oval, however, is placed in the same direction as that of the axis, as in the Pennatulæ before described.

LECTURE IX.

SKELETON OF ZOOPHYTES.

IN the preceding Lecture I described the structure of the skeleton of some of the Asteroid Zoophytes, and I have now to make a few further remarks upon the nature of the axis in two of the species to which I have already alluded.

The structure of the axis of the Gorgoniada has been the subject of controversy ever since the time of Ellis. Many authorities consider it to be inorganic, there can, however, be no doubt that although the polypes do not form the axis, they are mainly concerned in preserving its vitality, and as long as the polypes are alive, changes both in the interior and on the exterior of the axis are continually going on. In those axes having a horny stem the increase takes place by the addition of concentric lamina without much external

alteration; in the axis of Isis hippuris, however, the horny portion of the base, where no polypes exist, is gradually converted into calcareous material. In Isis, now Melitaa ochracea (Fig. 66, A), the changes are very remarkable; the axis, although jointed, has only a small portion of horny matter entering into its composition, it is almost wholly made up of spicula; the joints are easily cut with a knife, but the other parts are much harder. All the joints are composed of spicula of a yellow colour arranged in a peculiar form of net-work, as shown at c; the intermediate harder parts are made up of red-coloured spicula, internally like those of the joint, but externally, as shown at B, these have coalesced to form a solid mass; the same thing has taken place in the Red Coral, and spicula can be recognized in some portion or other of its substance, and although in all the hardest parts a crystalline structure appears, there is always a trace of cells to be seen.

In one species of Red Coral the cells of the axis are disposed more or less concentrically, the cells themselves being of small size, with their walls still visible; in another species the walls have disappeared, but the concentric laminæ indicating the successive stages of growth are very evident, and beautifully arranged in undulating lines. The organic basis of these axes was known to Hunter, and was more especially noticed by the late Mr. Hatchett, who so long ago as the year 1800 prepared a specimen illustrative of this fact,