69 



mass of cilia; but the fact that it is always distinctly separated from 

 the epithelium of the archenteron would seem to point towards its being 

 more probably some sUmy secretion. Anyhow, it is very useful for iden- 

 tifying the entoderm in the sections. 



jAt the age of about 42 hours the embryos have reached their full shape, 

 which is, indeed, most surprising for an Echinoid larva (PI. XVII. Figs. 

 5^6, 8). It is a simple, elongated, worm-shaped body, rounded at the 

 anterior end, truncated at the posterior end. There is nothing to remind 

 one of a Pluteus. A little below the middle there is generally a ringshaped 

 widening; in the prehminary notice I have said that this might perhaps 

 represent a rudiment of the postoral larval processes. After a closer exam- 

 ination of the larva I do not see any sufficient reason for this sup- 

 position. If rudiments of the larval skeleton could be found at this widen- 

 ing, that would serve to support the said suggestion; but it appears 

 that there is no trace of a larval skeleton. The body is covered by a 

 uniform ciliation, not provided with distinct ciliated bands, as it is 

 otherwise generally found in the Echinoderm-larvse of the simple worm- 

 shaped type (Crinoids, Dendrochirote Holothurians). In a somewhat later 

 stage there may, however, be found a rather distinct ciliated band on 

 the ringshaped widening (PI. XVII. Fig. 8) and also the oral end may 

 show an indication of a ciliated ring; but this appears to be no constant 

 feature. Red pigment also begins to appear about this time. It is especially 

 prominent round the oral end; in the later stages it may form a very 

 prominent ring just above the widening, and also in the anterior part 

 of the body it may be more or less distinctly arranged in rings (PI. XVII. 

 Figs. 10, 12, 14). Scattered pigment cells otherwise are found all over 

 the body. Some of the embryos have scarcely any pigment at all; these 

 latter probably give rise to the very light-coloured specimens which are 

 often found among the otherwise generally dark coloured specimens of 

 this species. 



jThe first indication of the metamorphosis was observed in embryos 

 42 hours old. Below the thickened wall a slight invagination is seen 

 (PI. XVII. Figs. 5, 6), which gradually widens so as to go nearly round 

 the body^ (PI. XVII. Fig. 8). This represents the amniotic invagination; 

 being at first a simple fold (PI. XVIII. Fig. 4) it gradually develops into 

 a true amniotic cavity, which, however, apparently never closes com- 

 pletely, a small pore remaining open (PI. XVIII. Figs. 5 — 6). Contempo- 

 raneously the hydrocoel differentiates and the five primary tubefeet are 

 formed (PI. XVII. Fig. 11). These, together with the first spines, formed 

 also by this time, protrude into the amniotic cavity (PI. XVIII. Figs. 5 — 6) 

 and soon push out through the opening, which widens more and more. 



