VIII 



ECHINODERMA TA ONTOGENY 



537 



2. The Free-swimming Larva (Figs. 402 (p. 510), 446, 447, 448). 



The external form of the free-swimming larva has already been described on 

 p. 510. 



The duration of this manner of life differs greatly in individuals of the same 

 brood, varying from a few hours to several days. 



Ectoderm. For the ciliated bands, see above, p. 510. 



In the intermediate zones, which are free from cilia, a fine cuticle becomes 

 differentiated from the ectodermal epithelium, the cells of which later begin to 

 secrete a homogeneous substance which separates cell from cell, so that the epi- 

 thelium comes to resemble a connective tissue. 



The neural plate, the neural tuft, and with them the larval nervous system attain 

 their highest development at this stage, but undergo complete degeneration in the 

 next. The ganglion cells below the neural plate 

 increase in number, and the layer of nerve fibres 

 spreads out over the whole anterior end of the larva. 

 Fine nerve trunks run to the ciliated rings, and two 

 specially strong ventral nerve trunks run back at 

 the sides of the vestibular imagination, their an- 

 tei'ior parts being beset with isolated ganglion cells. 



The adhesive pit becomes larger and deeper, 

 and towards the end of this period loses its ciliation 

 and assumes a glandular character. 



The vestibular invagination spreads over the 

 greater part of the ventral side. It closes and 

 becomes a tube, the lateral edges of the invaginatiou 

 growing towards one another and fusing in the 

 median line. This process takes place from behind 

 forwards, and is not fully completed during this 

 period, a small aperture being retained anteriorly. 

 The ciliation of the vestibule disappears. 



The intestine alters its shape. It spreads out 

 somewhat, assuming first the form of a hollow plate, 

 with the concavity directed ventrally and the con- 

 vexity dorsally. In the ventral concavity lies the 

 hydrocoel vesicle. At a later stage the intestine 

 again becomes rounded and vesicular. 



The two enteroccel sacs continue to change their 

 positions and to spread out. The right vesicle pro- 

 duces anteriorly five tubular outgrowths, which become grouped round the principal 

 axis. These five tubes arise, widened like funnels, from the right coelom, then 

 narrow anteriorly, and, losing their lumina, run out as strands. They are the 

 rudiments of the chambered sinus. 



The skeletal pieces of the stalk are at this time horse-shoe-shaped, and tend to 

 enclose the five tubes of the chambered sinus. When they become complete rings 

 the chambered sinus passes through them. 



The hydroccel vesicle becomes completely constricted from the parietal sinus, 

 flattens 'in the dorsoventral direction, and at once assumes the horse-shoe shape. 

 The gape of the horse-shoe points at first backwards and to the left, and finally to 

 the left and forwards. Five ventrally directed outgrowths appear on it, out of each 

 of which, at a later stage, three tentacle vessels arise. 



FIG. 44ti. Free-swimming larva 

 of Antedon, fifty-two hours old, 

 from the left side (after Seeliger). 

 I-V, The five ciliated rings ; 1, the 

 parietal sinus ; 2, the vestibule, 

 already closed posteriorly ; 3, the 

 hydrocoel ; 4, the hydropore ; 5, left 

 enterocoel vesicle ; 6, intestine ; 

 7, right enterocoel. 



