It is evident then that the individual Zoophyte, like the individual branch, is propagated by gemmation while the species is reproduced by ova.

The second order, Asteroida, is so named from the resemblance the polypes bear to a star. The polypes in this tribe have invariably eight tentacles, and the skeleton is composed either of horn or calcareous matter, in the form of a central axis or shaft, surrounded by the external flesh of the animal. The order contains three families, the Gorgoniada, Pennatulida, and the Alcyonidæ. I commence with the Gorgoniade, because the structure of the skeleton of this family is easily understood, and forms a good key to the comprehension of that in the other two families. The Gorgoniada are not only remarkable for elegance of shape, but some of them for beauty of colour; when branched, as in Isis hippuris, Fig. 64, a, they are called Sea-shrubs; but when all the branches are joined together in a reticulated manner, as in Gorgonia flabellum, they have received the name of Sea-fans. Whatever be the shape of the Polypidom, three parts require special notice-viz.: the central axis, the fleshy crust which surrounds it, and the polypes. As in this course of lectures our attention is confined to

the structure of the skeleton, the polypes demand and can receive only a passing notice. I may here mention that if any specimen still retaining the fleshy crust, be examined, a number of minute depressions will be observed, in which the polypes were located during the life of the animal; these polypes have eight tentacula, and at their base are a few tuberculated spicula, to which I shall advert when speaking of the Alcyonida. The central axis of most of the Gorgoniada is composed of horn, which in some species is of yellow or brownish colour, in others black; these last formerly received the generic name of Antipathes. In one species-G. Americana, Fig. 63, c-the axis consists of horny matter, in which spicula are imbedded in concentric circles; in Gorgonia petechialis, Fig. 63, A B, it is principally composed of spicula so arranged

A B, segment of a transverse section of the axis of Gorgonia petechialis. b, spicula from the centre. c, spicula from the margin. c, segment of a transverse section of the axis of Gorgonia Americana. a, spicula from the crust of G. Americana. d, spicula from the axis of the same.

as to leave a number of large canals which run from one end of the specimen to the other, and the spicula near the surface are much larger than those in the centre. In the Red Coral, Fig. 67, A, as well as in the beautiful pink Oculina rosea, the axis, D, is composed of laminæ of dense calcareous matter, exhibiting occasionally traces of spicula and of the cells in which the carbonate of lime was secreted. In the genus Isis the axis is made up of alternate joints of horny and calcareous matter; this curious structure is well seen in the Isis hippuris, Fig. 64, a, b, so named

a, portion of a branch of Isis hippuris, showing the crust with the apertures in which the polypes were lodged. b, axis composed of alternate layers of horn and calcareous matter. c, transverse section of the calcareous part of the axis. d, transverse section, through a joint at b, that has been covered with calcareous matter. e, e, e, spicula of the crust.

from its resemblance to the Equisetæ, or Horse Tails. The axis in some of these Zoophytes is perfectly smooth on its exterior surface, in others slightly grooved, as in the Red Coral, Fig. 67, A, whilst a few species show certain depressions which indicate the situation of the polypes, and some specimens, are covered with minute spines.

There are considerable differences in the structure of the horny axis of the Gorgonia; some are composed simply of concentric laminæ of brown horn, others as G. spiralis, Fig. 65, show not only concentric

FIG. 65.

laminæ of horn, but a number of large radiating lines which look like tubes, but are in reality connected with the spines seen on the surface; in others, as Gorgonia Americana, Fig. 63, c, the axis is composed of horny matter having numerous short spicula of the shape represented by d, imbedded in its substance, section of the axis of Gorgonia which are disposed in a somewhat concentric manner. The

Segment of a transverse

axis of Melitaa ochracea, Fig. 66, A, is jointed like that of Isis hippuris, but the horny matter is so small in quantity as only to suffice to hold together the numerous spicula of which the calcareous part of the axis is composed. These spicula are of a reddish colour in the more solid parts of the axis, those on the surface being larger and

darker than those in the centre, as shown at B; but those in the joints are of a yellow colour and smaller,

A, axis of Melitaa ochracea. a, a, joints in the axis. B, portion of a transverse section of the axis made through the most solid part. c, transverse section of the axis made through a joint.

as shown at c. The axis of the Corallium rubrum, or red coral, is very dense, and capable of taking a high polish; it is even manufactured into beads, bracelets, and other ornaments, although it every here and there exhibits traces of being formed of spicula; these, however, are shorter and more completely blended together than they are in M. ochracea, and there is in addition a concentric laminated arrangement, as shown in