16 EMBRYOLOGY OF THE STAEFISH. 



two water-tubes present, but I bave also seen in Starfisbes and Opbiurans, 

 as be bas well sbown in Opbiurans alone, that tbe wbole rosette of tbe future 

 ambulacral system is developed only upon tbe surface of one of tbese, tbe 

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

 madreporic body.] 



Appearance of the Chords of vihratile Cilia. — Tbe cilia, spreading over the 

 wbole surface, which moved tbe 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 (PI. II. Fig. 20), the larva 

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

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

 become still more sluggish ; it is then that we find tbe first aj^pearance 

 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 tbe greater bending and extending of tbe oesophagus and alimentary 

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

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

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

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

 their only means of locomotion (PI. II. v, v, Figs. 20-28). Tbese 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 tbe depression in which 

 tbe mouth is placed [v, v', PI. II. Fig. 21). 



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

 midergone but slight modifications, tbe changes taking place principally in 

 the digestive organs. Tbe phases through which tbe larva passes in the 

 next three days are of a very different character ; the alimentary canal, 

 the stomach, and the oesophagus become more circumscribed by the 

 increasing difference noticeable in tbe walls of these regions. Tbe stom- 

 ach {cl) is always marked by the greater thickness of its 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 increase in tbe growth of tbe water-tubes 

 {w, tv), which by tbe end of tbe sixth day (PI. II. Figs. 27, 28) extend 

 as far as the corners of the mouth and along tbe edge of tbe walls of 

 tbe stomach, towards the anal extremity (PL II. Figs. 24, 26, tv, w'). 

 When viewed in profile (PI. II. Figs. 25, 27), it will be seen that the 



