The later development of the mouth, and of the ciliated ridge surrounding it, is involved in some obscurity. It appears from Metschnikoff (No. [560]) that an invagination of the œsophagus takes place, carrying with it the ciliated ridge around the mouth. This ridge becomes eventually converted into the covering for the five tentacular outgrowths of the water-vascular ring ([fig. 258]), and possibly also forms the nervous system.
The opening of the œsophageal invagination is at first behind the foremost ciliated ring, but eventually comes to lie in front of it, and assumes a nearly terminal though slightly ventral position ([fig. 258]). No account has been given of the process by which this takes place, but the mouth is stated by Metschnikoff (though Müller differs from him on this point) to remain open throughout. The further changes in the metamorphosis are not considerable. The ciliated bands disappear, and a calcareous ring of ten pieces, five ambulacral and five interambulacral, is formed round the œsophagus. A provisional calcareous skeleton is also developed.
All the embryonic systems of organs pass in this case directly into those of the adult.
Fig. 257. Three stages in the development of Synapta. A and B are viewed from the ventral surface, and C from the side. (After Metschnikoff.)
m. mouth; oe. œsophagus; pv. walls of the perivisceral cavity; wv. longitudinal vessel of the water-vascular system; p. dorsal pore of water-vascular system; cr. ciliated ring formed round the mouth from parts of the primitive ciliated band.
The metamorphosis of most Holothuroidea is similar to that just described. In Cucumaria (Selenka) there is however no Auricularia stage, and the uniformly ciliated stage is succeeded by one with five transverse bands of cilia, and a præ-oral and an anal ciliated cap. The mouth is at first situated ventrally behind the præ-oral cap of cilia, but the præ-oral cap becomes gradually absorbed, and the mouth assumes a terminal position.
In Psolinus (Kowalevsky) there is no embryonic ciliated stage, and the adult condition is attained without even a metamorphosis. There appear to be five plates surrounding the mouth, which are developed before any other part of the skeleton, and are regarded by P. H. Carpenter (No. [548]) as equivalent to the five oral plates of the Crinoidea. The larval condition with ciliated bands is often spoken of as the pupa stage, and during it the larvæ of Holothurians proper use their embryonic tube feet to creep about.