By the change in position of the original ventral surface relatively to the axis of the body, the bilateral symmetry of the larva passes into a radial symmetry. While the first skeletal elements of the calyx are being formed, the skeleton of the stem is also established. The terminal plate is first of all established, then the joints, eight at first, of the stem. The centro-dorsal plate is stated by Thomson to be formed as the uppermost joint of the stem[227]. The larva, after the completion of the above changes, is shewn in [fig. 268] B, and somewhat more diagrammatically in [fig. 269].
Fig. 270. Young Pentacrinoid larva of Antedon. (From Carpenter; after Wyville Thomson.)
1. terminal plate of stem; cd. centro-dorsal plate; 3. basals; 4. radials; or. orals.
After the above elements of the skeleton have become established the ciliated bands undergo atrophy, and shortly afterwards the larva becomes attached by the terminal plate of its stem. It then passes into the Pentacrinoid stage. The larva in this stage is shewn in [fig. 268] C and [fig. 270]. New joints are added at the upper end of the stem next the calyx, and a new element—the radials—makes its appearance as a ring of five small plates, placed in the space between the basals and orals, and in the intervals alternating with them ([fig. 270], 4). The roof of the oral vestibule (vide [fig. 253] and p. [551]) has in the meantime become ruptured; and the external opening of the mouth thus becomes established. Surrounding the mouth are five petal-like lobes, each of them supported by an oral plate ([fig. 268] C). In the intervals between them five branched and highly contractile tentacles, which were previously enclosed within the vestibule, now sprout out: they mark the position of the future radial canals, and are outgrowths of the water-vascular ring. At the base of each of them a pair of additional tentacles is soon formed. Each primary tentacle corresponds to one of the radials. These latter are therefore, as their name implies, radial in position; while the basals and orals are interradial. In addition to the contractile radial tentacles ten non-contractile tentacles, also diverticula of the water-vascular ring, are soon formed, two for each interradius.
In the course of the further development the equatorial space between the orals and the basals enlarges, and gives rise to a wide oral disc, the sides of which are formed by the radials resting on the basals; while in the centre of it are placed the five orals, each with its special lobe.
The anus, which is formed on the ventral side in the position of the blastopore (p. [551]), becomes surrounded by an anal plate, which is interradial in position, and lies on the surface of the oral disc between the orals and radials. On the oral plate in the next interradius is placed the opening of a single funnel leading into the body cavity, which Ludwig regards as equivalent to the opening of the madreporic canal (vide p. [551])[228].
From the edge of the vestibule the arms grow out, carrying with them the tentacular prolongation of the water-vascular ring. Two additional rows of radials are soon added.
The stalked Pentacrinoid larva becomes converted, on the absorption of the stalk, into the adult Antedon. The stalk is functionally replaced by a number of short cirri springing from the centro-dorsal plate. The five basals coalesce into a single plate, known as the rosette, and the five orals disappear, though the lobes on which they were placed persist. In some stalked forms, e.g. Rhizocrinus Hyocrinus, the orals are permanently retained. The arms bifurcate at the end of the third radial, and the first radial becomes in Antedon rosacea (though not in all species of Antedon) concealed from the surface by the growth of the centro-dorsal plate. An immense number of funnels, leading into the body cavity, are formed in addition to the single one present in the young larva. These are regarded by Ludwig as equivalent to so many openings of the madreporic canal; and there are developed, in correspondence with them, diverticula of the water-vascular ring.