Fig. 263. Diagrammatic Figures Shewing the Evolution of Echinoid Plutei. (Copied from Müller.) The calcareous skeleton is not represented. E. Pluteus of Spatangus.
m. mouth; an. anus; d. anterior arms; d´. point where lateral arms arise in the Ophiuroid Pluteus; e. anterointernal arms; e´. posterior arms; g´. anterolateral arms; g. anteroexternal arms.
The ventral surface of the permanent Ophiuroid is formed by the concave surface between the mouth and anus. Between this and the stomach is situated the water-vascular ring. It is at first not closed, but is horseshoe-shaped, with five blind appendages ([fig. 262]). It eventually grows round the œsophagus, which, together with the larval mouth, is retained in the adult. The five blind appendages become themselves lobed in the same way as in Asterias, and grow out along the five arms of the disc and become the radial canals and tentacles. All these parts of the water-vascular system are of course covered by skin, and probably also surrounded by mesoblast cells, in which at a later period the calcareous plates which lie ventral to the radial canal are formed. The larval anus disappears. As long as the larval appendages are not absorbed the ventral and dorsal discs of the permanent Ophiuroid fit as little as in the case of the Brachiolaria, but at a certain period the appendages are absorbed. The calcareous rods of the larval arms break up, the arms and anal lobe become absorbed, and the dorsal and ventral discs, with the intervening stomach and other organs, are alone left. After this the discs fit together, and there is thus formed a complete young Ophiuroid.
The whole of the internal organs of the larva (except the anus), including the mouth, œsophagus, the body cavity, etc. are carried on directly into the adult.
The larval skeleton is, as above stated, absorbed.
The viviparous larva of Amphiura squamata does not differ very greatly from the larvæ with very imperfect arms. It does not develop a distinct ciliated band, and the provisional skeleton is very imperfect. The absence of these parts, as well as of the anus, mentioned on p. [549], may probably be correlated with the viviparous habits of the larva. With reference to the passage of this larva into the adult there is practically nothing to add to what has just been stated. When the development of the adult is fairly advanced the part of the body with the provisional skeleton forms an elongated rod-like process attached to the developing disc. It becomes eventually absorbed.
Fig. 264. Two larvæ of Strongylocentrus. (From Agassiz.)
m. mouth; a. anus; o. œsophagus; d. stomach; c. intestine; v´. and v. ciliated ridges; w. water-vascular tube; r. calcareous rods.