The original hinder peritoneal vesicle grows rapidly, and divides into two ([fig. 248] C, pl. and pr.), which encircle the two sides of the alimentary canal, and meet above and below it. The outer wall of each of them attaches itself to the skin, and the inner one to the alimentary canal and water-vascular system; in both cases the walls remain separated from the adjacent parts by a layer of the amœboid cells already spoken of. The cavity of the peritoneal vesicles becomes the permanent body cavity. Where the walls of the two vesicles meet on the dorsal side, a mesentery, suspending the alimentary canal and dividing the body cavity longitudinally, is often formed. In other parts the partition walls between the two sacks appear to be absorbed.
The amœboid cells, which were derived from the invaginated cells, arrange themselves as a layer round all the organs ([fig. 249]). Some of them remain amœboid, attach themselves to the skin, and form part of the cutis; and in these cells the calcareous spicula of the larva and adult are formed. Others form the musculature of the larval alimentary tract, while the remainder give rise to the musculature and connective tissue of the adult.
The development of the vascular system is not known, but the discovery of Kowalevsky, confirmed by Selenka, that from the walls of the water-vascular system corpuscles are developed, identical with those in the blood-vessels, indicates that it probably develops in connection with the water-vascular system. The observations of Hoffmann and Perrier on the communication of the two systems in the Echinoidea point to the same conclusion. Though nothing very definite is known with reference to the development of the nervous system, Metschnikoff suggests that it develops in connection with the thickened bands of epiblast which are formed by a metamorphosis of the ciliated bands of the embryo, and accompany the five radial tubes (vide p. [555]). In any case its condition in the adult leaves no doubt of its being a derivative of the epiblast.
From the above description the following general conclusions may be drawn:—
(1) The blastosphere stage is followed by a gastrula stage.
(2) The gastrula opening forms the permanent anus, and the mouth is formed by a fresh invagination.
(3) The mesoblast arises entirely from the invaginated cells, but in two ways:—
(a) As scattered amœboid cells, which give origin to the muscles and connective tissue (including the cutis) of the body wall and alimentary tract.
(b) As a portion separated off from the archenteron, which gives rise both to the epithelioid lining of the body cavity, and of the water-vascular system.
(4) The œsophagus is derived from an invagination of the epiblast, and the remainder of the alimentary canal from the archenteron.