ON THE EMBRTOLOGT OF ECHINODERMS. 5 



of the two water-tubes w, w', is quite striking. The left water-tube [w') — left when 

 seen from the aboral side, the anal extremity being turned down as in these figures — 

 is fast pushing through the mass of the larva, and finds its way to the surface at about 

 the condition repi'esented in fig. 15, where the water-pore, the madreporic body, allows 

 the water to enter freely into this water-tube. In a somewhat more advanced larva, 

 seen obliquely from the oral side, fig. 10, we can trace the mode of development of the 

 chord of vibratile cilia ; it is formed of a single continuous line, extending round the 

 mouth ; it forms bvit a single shield, and ijot two as in the Starfish, where the first 

 trace of this chord is the appearance of two separate arcs forming eventually two dis- 

 tinct plastrons. The little larva looks in this condition like a quadrangular pyramid 

 W'ith a rounded apex and rounded angles at the base. The corners of the base e, e'" 

 are the origin of the first arms of the larvae (figs. 9, 10). Owing to the great increase 

 of the dorsal and oral parts of the larvEe, they change their general appearance very 

 rapidly. (See figs. 11, 12.) As the intestine becomes more distinct from the stomach, 

 the angle which their axes make grows more acute (fig. 11 c, d)\ the mouth (ni) is 

 removed further from the anus. The walls of the oesophagus [o) are now capable of 

 considerable expansion and contraction ; they are much thinner than those of the 

 intestine or stomach. Fig. 12, which is fig. 11 seen from the oral side, shows the 

 course of the vibratile chord, the position of the arms e\ e'", the great size of the rods 

 (/), with their branches, and the difference of level between the opening of the mouth 

 and anus. From an examination of figs. 11 and 12 the position of the rods can be 

 determined, one main part extending from the anal extremity to the arms e', another 

 extending in a curved line (fig. 11) from e' to e"\ and sending off" a small branch which 

 runs between the anus and the digestive cavity (fig. 12). This will, perhaps, be more 

 clear on examining the larva in such a way (fig. 13) as to bring the vibratile chord into 

 the field ; this stage does not diflfer materially from that represented in fig. 10, the 

 changes which have taken place defining the arms more sharply by the indentations of 

 the vibratile chord. The intestine, the stomach, and the oesophagus are clearly dis- 

 tinguished by the different character of their walls. The water-tubes are not united, 

 and have not increased in size. This condition presents a material diff'erence in the 

 degree of development when compared with the corresponding stage of a Starfish (fig. 

 11, Proc. Am. Acad.). Here the water-tubes occupy the most important portion of 

 the larva, while in the Sea-urchin larva the most striking characteristic is the amount 

 of room taken up by the stomach and oesophagus compared with that occupied by 

 the water-tubes. 



In the subsequent stages (figs. 14 and 15), the Echinus larvae have reached forms 



