Fig. 265.—View of Antedon rosacea from aboral surface, × 4. c, Centro-dorsal; cir, cirrus; R1, R2, R3, the three radial plates of one column; syz, syzygy.
It is evident, both from the number of ambulacral grooves and of the columns of radials, that Antedon has only five radii, and each pair of arms must be regarded as having arisen by the bifurcation of a primitive arm. This is proved to be true by a study of the development, and it can further be shown that the arms fork repeatedly; but in these further bifurcations one fork remains short, and forms a pinnule, whilst the other continues the arm. Thus the arm, instead of being a single axis, is really a series of axes—in a word, it is a "sympodium."
If in the case of any bifurcation the two forks were to develop equally, the number of arms in that ray would be doubled, and this actually happens in the case of other species of Antedon.
Digestive System.—The mouth leads through a short vertical oesophagus into an enlarged stomach, which lies horizontally curved around the axis of the calyx. The stomach is succeeded by a short intestine, which leads into the anal papilla. Both oesophagus and stomach are ciliated, and the food consists of minute organisms, swept into the mouth by the current produced by the cilia covering the ambulacral grooves and podia; the ten arms may indeed be compared to a net spread out in the water to catch swimming prey.
The water-vascular system consists of a ring closely surrounding the mouth, from which radial canals are given off which underlie the ambulacral grooves and bifurcate with them. The podia have no ampullae, but muscular strands traverse the cavities of the radial canals, and that of the ring-canal, and by their action water can be forced into the podia, which are thus extended. Numerous stone-canals hang down from the ring-canal, and open freely into the coelom; they do not, as in Holothuroidea (where the same arrangement occurs), end in sieve-like madreporites. The tegmen, i.e. the ventral surface of the calyx, is pierced by a number of isolated pores lined by ciliated cells, which suck in water. In the oldest Pelmatozoa there seems to have been a regular madreporite. In the larva of Antedon there is but one pore-canal, which, as in most Eleutherozoa, leads into a special section of the coelom, the "axial sinus," embedded in the body-wall, with which also the single stone-canal communicates; but later the division between the axial sinus and the rest of the coelom breaks down, and then the pore-canals and stone-canals become multiplied independently of each other (Fig. 266, m.p, p.c, and st.c).
Nervous System.—In the young stalked form the nervous system, as in other Echinoderms, consists of a ring round the mouth, from which radial cords are given off which run under the ambulacral grooves (Fig. 266, nerv.rad.v).
The fibres of this nervous system are, as in Asteroidea, immediately beneath the bases of the ectoderm cells. A large band of fibres is given off to each podium, which is covered with minute elevations, each with pointed sense-hairs in the centre. As the animal grows, another nervous system makes its appearance, which is developed from the coelomic wall, the cells of certain tracts of which multiply and bud off ganglion cells from which the fibres grow out.
Fig. 266.—Diagrammatic longitudinal section through one arm and the opposite interradius of Antedon. ax, Central canal of centro-dorsal, with prolongation of genital stolon; B, rosette, consisting of coalesced basals; Br1, Br2, Br3, Br4, the first four brachial ossicles; chamb, chambered organ; coel.coe, coelomic canal of arm; gen.coe, genital canal of arm; gen.r, genital rachis; gen.st, genital stolon; m.p, madreporic pores; musc.long, longitudinal muscle; nerv.rad.d, dorsal radial nerve; nerv.rad.v, ventral radial nerve; p.c, pore-canal; pod, podium; R1-R3, 1st to 3rd radials; st.c, stone-canal; sub.coe, sub-tentacular canal of arm; w.v.r, radial water-vessel.