Crinoidea. The larva of Antedon, while still within the egg-shell, assumes an oval form and uniform ciliation. Before it becomes hatched the uniform layer of cilia is replaced by four transverse bands of cilia, and a tuft of cilia at the posterior extremity. In this condition it escapes from the egg-shell ([fig. 268] A), and becomes bilateral, owing to a flattening of the ventral surface. On the flattened surface appears a ciliated depression corresponding in position with the now closed blastopore (vide p. [550]). The third ciliated band bends forward to pass in front of this ([fig. 269]). Behind the last ciliated band there is present a small depression of unknown function, also situated on the ventral surface. The posterior extremity of the embryo elongates to form the rudiment of the future stem, and a fresh depression, marking the position of the future mouth, makes its appearance on the anterior and ventral part.
Fig. 268. Three stages in the development of Antedon (Comatula.)
(From Lubbock; after Thomson.)
A. larva just hatched; B. larva with rudiment of the calcareous plates; C. Pentacrinoid larva.
While the ciliated bands are still at their full development, the calcareous skeleton of the future calyx makes its appearance in the form of two rows, each of five plates, formed of a network of spicula ([figs. 268] B and [269]). The plates of the anterior ring are known as the orals, those of the posterior as the basals. The former surround the left, i.e. anterior peritoneal sack; the latter the right, i.e. posterior peritoneal sack. The two rows of plates are at first not quite transverse, but form two oblique circles, the dorsal end being in advance of the ventral. The rows soon become transverse, while the originally somewhat ventral oral surface is carried into the centre of the area enclosed by the oral plates.
Fig. 269. Larva of Antedon with rudiments of calcareous skeleton. (From Carpenter; after Thomson.)
1. Terminal plate at the end of the stem; 3. basals; or. orals; bl. position of blastopore.