(1) The presence of the post-anal lobe at the hind end of the body. (2) The slight development of a præ-oral lobe. (3) The provisional calcareous skeleton in the larval arms.
Fig. 262. Pluteus larva of an Ophiuroid. (From Gegenbaur; after Müller.)
A. rudiment of young Ophiuroid; d´. lateral arms; d. anterior arms; e´. posterior arms.
Great variations are presented in the development of the arms and provisional skeleton. The presence of lateral arms is however a distinctive characteristic of the Ophiuroid Pluteus. The other arms may be quite absent, but the lateral arms never.
The formation of the permanent Ophiuroid takes place in much the same way as in the Asteroidea.
There is formed ([fig. 262]) on the right and dorsal side of stomach the abactinal disc supported by calcareous plates, at first only five in number and radial in position[223]. The disc is at first not symmetrical, but becomes so at the time of the resorption of the larval arms. It grows out into five processes—the five future rays. The original five radial plates remain as the terminal segments of the adult rays, and new plates are always added between the ultimate and penultimate plate (Müller), though it is probable that in the later stages fresh plates are added in the disc.
