Fig. 281.—Hemiaster philippi. Enlarged view of a single petal, showing the embryos in situ. (From Wyville Thomson.) The whole animal is shown in Fig. 250, p. [555].
In the typical larval development the eggs are fertilised after being laid, and they then undergo segmentation into a number of equal, or nearly equal, segments or "blastomeres." These arrange themselves in the form of a hollow sphere or "blastula," the cavity of which is called the "blastocoel" and afterwards becomes the primary body-cavity of the larva. This cavity contains an albuminous fluid, at the expense of which development appears to be carried on (Fig. 282, B). The cells forming the blastula acquire cilia, and the embryo begins to rotate within the egg-membrane, which it soon bursts, and, rising to the surface of the sea, begins its larval life. The blastula is therefore the first well-marked larval stage, and it is found in a more or less recognisable form in life-histories of members of every large group in the animal kingdom. Only in the case of Echinodermata and of forms still lower in the scale, however, does it appear as a larval stage. The free-swimming blastula stage is reached in from twelve to twenty-four hours. Soon the spherical form of the blastula is lost; one side becomes flattened and thickened, owing to a multiplication of cells, so that they become taller and narrower in shape. Shortly afterwards this thickened plate becomes buckled inwards, encroaching on the cavity of the blastocoel. The larva has now reached the second stage of its development; it has become a "gastrula" (Fig. 282, C). The plate of thickened cells has become converted into a tube called the "archenteron" (Fig. 282, C, arch), which is the rudiment of both the alimentary canal and the coelom of the adult. This tube communicates with the exterior, in virtue of its mode of formation, by a single opening which is called the "blastopore," which becomes the anus of the later larva and adult. Whilst the gastrula stage is being acquired, the blastocoel or primary body-cavity is invaded by wandering cells budded from the wall of the archenteron (Fig. 282, A, B, C, mes). These cells, which are called "mesenchyme," are the formative cells of the skeleton, connective tissue, and wandering cells of the adult. When the larva has a skeleton they are formed very early, arising in the young blastula stage (Ophiuroidea) or in the stage immediately before the formation of the archenteron (Echinoidea, Fig. 282, A, B) and secreting the skeleton. When the larva is devoid of a skeleton (Asteroidea and Holothuroidea), the mesenchyme usually does not appear till the gastrula is fully formed.
Fig. 282.—Echinus esculentus. A, optical section of living blastula. B, section of preserved blastula. The network of strings in the interior is the result of the coagulation of the albuminous fluid. C, section of gastrula. arch, Archenteron; mes, mesenchyme cells, attached by protoplasmic strands to the wall of the embryo. × 150.
The gastrula stage is reached in twenty to thirty-six hours. Then one side of the larva becomes concave, and the cilia become restricted to a thick band surrounding this area. In this way is formed the rudiment of the longitudinal band of cilia, which is the organ of locomotion throughout the larval life. At the apex of the archenteron a thin-walled vesicle is formed, which soon becomes divided off from the rest. This vesicle, which almost immediately divides into two sacs, right and left, is the rudiment of the "coelom" or secondary body-cavity of the larva; the remainder of the archenteron forms the definitive gut, and becomes divided by constriction into an oesophagus, a stomach, and an intestine, and at the same time bent into a shallower or deeper V-shape, the concavity of which is towards the concave side of the body. Within this area of the surface a new funnel-shaped depression makes its appearance. This is the "stomodaeum," the rudiment of the mouth of the larva, and it soon joins the apex of the larval oesophagus; the conjoined tubes henceforth bearing the name oesophagus since the ectodermal and endodermal parts become indistinguishably fused. Along the sides and floor of the oesophagus is formed a V-shaped ridge bearing strong cilia; this is the "adoral band of cilia" which sweeps the food (consisting of Diatoms, Infusoria, etc.) into the mouth. The larva is now known as a Dipleurula and appears in four modifications, each characteristic of a Class of Eleutherozoa. These differ from one another principally in the following points:—(a) The folding of the ciliated band; (b) the divisions of the coelomic sacs; (c) the development and fate of the praeoral lobe (i.e. the part of the body in front of the mouth); (d) the fate of the larval mouth. The types of Dipleurula are as follows:—
Fig. 283.—Bipinnaria of Luidia. a, Anus; a.b, adoral ciliated band; a.c.o.b, anterior median process; a.d.a, anterior dorsal process; a.v.a, prae-oral process; m, mouth; p.c.o.b, median dorsal process; p.d.a, posterior dorsal process; p.l.a, posterior lateral process; p.v.a, post-oral process. (After Garstang.)
(1) The Bipinnaria, the larva of Asteroidea. In this type there is a very long prae-oral lobe. The ciliated band runs along its edges, and is produced into a backwardly directed loop on its under surface. This loop soon becomes separated from the rest of the band as a distinct prae-oral loop, the rest forming a post-oral loop. Both loops are drawn out into short tag-like processes, in which we may distinguish (following Mortensen's[[517]] notation) in the prae-oral loop an anterior median process (Fig. 283, a.c.o.b), and a pair of prae-oral processes (a.v.a). In the post-oral loop there is a median dorsal process (p.c.o.b) and paired anterior dorsal (a.d.a), posterior dorsal (p.d.a), posterior lateral (p.l.a), and post-oral (p.v.a) processes. At the apex of the prae-oral lobe between prae-oral and post-oral ciliated rings there is an ectodermic thickening, recalling the so-called apical plate of Annelid larvae.
Fig. 284.—A, Ophiopluteus of Ophiothrix fragilis. hy, Hydrocoel; l.p.c, left posterior coelom; oes, oesophagus; r.p.c, right posterior coelom; st, stomach. B, metamorphosis of Ophiopluteus of Ophiura sp. (After Johannes Müller.)