In Auricularia (Fig. 287, D) the coelomic vesicle, after the pore-canal is formed, divides into an anterior and a posterior half. The posterior part then divides into right and left halves, whilst the anterior sac divides into dorsal and ventral halves, connected by a narrow neck. The ventral half soon assumes the familiar feature of the hydrocoel (Fig. 287, D3, l.hy), whilst the dorsal half forms an insignificant swelling on the course of the conjoined stone- and pore-canals, which represents the left anterior coelom of the other types; neither right anterior coelom nor right hydrocoel being developed. The neck of communication between dorsal and ventral halves is, of course, the stone-canal.
The Dipleurula larva leads a free-swimming life for a period varying from two weeks to two months, and then undergoes metamorphosis into the adult form. The details of this process have been worked out in comparatively few cases; and the species in which they are most thoroughly known is the Asteroid Asterina gibbosa. The development of this species is intermediate in character between the embryonic and larval types. The eggs are larger than is usual among Asteroidea, and are filled with a bright orange yolk. The larva differs from the Bipinnaria in the absence of the characteristic ciliated bands and in the very early occlusion of the anus. There is, however, a band of cilia round the edge of the prae-oral lobe, which corresponds to portions of the prae-oral and post-oral bands combined of the Bipinnaria.[[519]]
The larva has a form which may be described as boot-shaped (Figs. 288, 289). The sole of the boot is the great prae-oral lobe, behind which is the mouth. The larva takes little or no food, and completes its metamorphosis in ten to twelve days. It does not swim at the surface, but creeps slowly over the bottom by the aid of the ciliated band mentioned above, while it can also attach itself, using the edges of the prae-oral lobe as a sucker.
After leading an existence of this kind for seven or eight days it fixes itself permanently by a disc-like prominence, which appears on the anterior surface of the prae-oral lobe within the area surrounded by the thickened rim which, as explained above, forms a margin to the prae-oral lobe. The larva then becomes divided by a constriction into a disc and a stalk, and the former is gradually converted into the body of the young Starfish, whilst the latter continually diminishes in size, and eventually entirely disappears, when the young Starfish commences to walk about on its podia. The disc becomes bent downwards and to the left, so as to make nearly a right angle with the stalk, and the last vestige of the latter springs from the peristome of the Starfish inside the water-vascular ring (Figs. 289, B, C).
Fig. 288.—Fully grown larval stages of Asterina gibbosa. A, fully grown larva; B, left, and C, right view of a larva seven days old in the beginning of the metamorphosis. m, Mouth; 1-5, the five lobes of the hydrocoel; I.-V., the rudiments of the arms. (After Ludwig.) × 45.
The form of the Starfish is attained principally by the preponderant growth of the left hydrocoel and of the left posterior coelom. Both these sacs take on the form of hoops, which, by the meeting of their ends, are converted into rings. The hydrocoel has already grown out into five lobes, which are the rudiments of the radial water-vascular canals, and the tips of which become the terminal sensory tentacles (Figs. 288, 289, 1-5); but now the left posterior coelom grows out into five lobes also, forming a parallel but outer ring. These lobes (Figs. 288, 289, I.-V.) are the rudiments of the arms, which are at first quite independent of those of the radial canals, but gradually, when the larva has attained the age of nine days (Fig. 289, B, C), they become applied to the outgrowths of the hydrocoel. These by this time have developed each two pairs of branches, the rudiments of the first two pairs of tube-feet in each radius. The larval mouth vanishes, and a new mouth is formed on the left side in the centre of the hydrocoel ring, when the metamorphosis is complete. The adult anus is formed about the same time. The primary pore-canal in Asterina as in all Dipleurulae opens into the anterior coelom; the stone-canal is formed from a ciliated groove running along the neck of communication between this and the hydrocoel. The constriction dividing the body into disc and stalk divides the anterior coelom (single in Asterina as in the older Bipinnaria) into two parts; the portion included in the disc forms the axial sinus of the adult. The lower end of the axial sinus expands and surrounds the adult mouth, forming the inner perihaemal ring; the outer perihaemal ring is formed by the juxtaposition of four wedge-shaped outgrowths from the left posterior coelom and one from the anterior coelom. From these the radial perihaemal canals subsequently grow out into the arms. The metamorphosis of Bipinnaria has been well worked out by Goto,[[520]] and it agrees in essential features with that of Asterina gibbosa; in fact, the differences which Goto maintains between the two types may be reasonably explained on the supposition of some stages having escaped the notice of this observer. The larva develops on the apex of the prae-oral lobe three papillae for occasional attachment,[[521]] and in the centre of these a cup-shaped disc for permanent fixation when the prae-oral lobe is converted into a stalk. When these papillae (Fig. 290, fix) have been developed the larva is known as a Brachiolaria.
Fig. 289.—Views of larvae of Asterina gibbosa in the course of metamorphosis. A, larva of eight days, from the right; B, left, and C, right view of larva of nine days; 1-5, lobes of hydrocoel; I-V, rudiments of arms. (After Ludwig.) × 45.
