16 EMBRYOLOGY OF THE STARFISH. 



two water-tubes present, but I have also seen in Starfishes and Ophiurans, 

 as he has well shown in Ophiurans alone, that the whole rosette of the future 

 ambulacral system is developed only upon the surface of one of these, the 

 one communicating with the exterior through the dorsal pore, the future 

 madreporic body.] 



Appearance of the Cliords of vibratile Cilia. — The cilia, spreading over the 

 whole surface, which moved the embryo so rapidly at first, have almost 

 entirely disappeared, and are no longer capable of propelling such a large 

 mass; consequently, at this last-mentioned stage (PL II. Fig. 20), the larva 

 is very sluggish, advancing but little, and rotating slowly about a longi- 

 tudinal axis at the same time. During the third day, the movements 

 become still more sluggish ; it is then that we find the first appearance 

 of the organs which are to propel the larva in future. The general out- 

 line does not change during the third day; the principal transformations 

 are the greater bending and extending of the oesophagus and alimentary 

 canal, the increase in size of the mouth, of the water-tubes, and the 

 appearance of slight projections, small clusters of vibratile cilia, near the 

 anterior and posterior sides of the mouth, which are the beginning of 

 rows, extending in older larva? in continuous lines all round the body, and 

 their only means of locomotion (PI. II. v, v, Figs. 20-28). These rows 

 are at first two very short arcs (v, v\ PI. II. Fig. 22), with their convexi- 

 ties placed opposite one another on each side of the depression in which 

 the mouth is placed (v, v, PI. II. Fig. 21). 



The general outline of the larva has, up to this stage (PI. II. Fig. 20), 

 undergone but slight modifications, the changes taking place principally in 

 the digestive organs. The phases through which the larva passes in the 

 next three days are of a very difierent character; tlie alimentary canal, 

 the stomach, and the oesophagus become more circumscril^ed by the 

 increasing diflerence noticeable in the walls of these regions. The stom- 

 acii {(I) is always marked by the greater thickness of it^ walls ; while, 

 with increasing age, the walls of the oesophagus {o) become more attenu- 

 ated, and capable of greater expansion and contraction (PI. II. Figs. 25, 

 0, 27). We observe, also, a rapid ineroase in the growth of the water-tubes 

 [iv, w\ which by the end of the sixth day (PI. II. Figs. 27, 28) extend 

 as far as tlie corners of the luoiilli aiul alouir the edj^e of the walls of 

 tlie stomach, towards the anal extremity (PI. 11. Figs. 24, 2G, w, «•'). 

 When viewed in prufde (PI. 11. Figs. 2o, 27), it will be seen that the 



