Page:Proceedings of the Royal Society of London Vol 69.djvu/290

274 wedges, which alternate with the five radial canals which have been developed from the hydrocoele (fig. 7). These outgrowths soon lose

Fig. 7. Diagrammatic Section of the developing Oral Disc of the future Echinus in a Larva 26 days old.

Names as before. In addition, am., amniotic cavity ; inv., invaginations of coelomic epithelium, which are the rudiments of the periheemal spaces and of the " lantern ccelom."

their connection with the ccelom, and become converted into closed sacs. From each of them a narrow outgrowth sprouts out, which insinuates itself between the water-tube and the ectoderm of the base of the tentacle. In this way the radial perihsemal canals are formed. The main body of the sac, however, forms a tooth-sac (fig. 8). From its floor an upstanding projection is formed, which is the root of the tooth, whilst from its sides the alveoli, which constitute the jaw, are formed. Calcification, however, proceeds in these parts only to a very slight extent till after the metamorphosis.

By the time the larva has attained an age of 32—38 days, the primary tube-feet have become long, arid have acquired their suckers, and, in addition, a curious little sensory prominence, consisting of elongated ectoderm cells situated in the centre of the sucker ; the three pedicellarise mentioned above are fully developed, and the four primary ciliated epaulettes have grown so as to almost meet one another, and thus form a complete circle. In addition, two secondary ciliated epaulettes have been formed in a more posterior position (see fig. 5) ; these extra epaulettes are characteristic of Echinus esculentus as opposed to Echinus miliaris.

The larva continues to grow up to 36 38 days after fecundation. By that time several important changes have taken place. The hydro-