392 



ZOOLOGY 



SECT. 



which represents one of the radial parts of the ambulacral system, 

 the horse-shoe itself representing the ring-vessel. The rudiment 

 of the madreporic canal arises as a groove on the posterior wall of 

 the anterior coelome. This develops into a canal leading from the 

 hydrocoele to the anterior coelome, and eventually entering into 

 connection with the dorsal pore, forms a tube, the madreporic 

 canal, leading from the ring canal to the madreporite, of which the 

 dorsal pore represents the first-formed aperture. 



As the hydrocoele develops, its form influences the external 

 shape of the larva ; on the left-hand side there grows out a five- 

 lobed elevation (Fig. 316, ami)), each of the lobes corresponding 

 to one of the five lobes of the hydrocoele. Each of the latter then 

 becomes divided, first into three rounded processes (Fig. 315, B, 

 amb), and then into five, and these project freely on the 



ABO 



3 31 



FIG. 318. Views of the larva of Asterina gibbosa in the course of metamorphosis. A, larva 

 of eight days, from the right ; B, left, and C, right view of the larva of nine days ; 1-5, lobes 

 of hydrocoele ; I-V, rudiments of arms. (From MacBride, after Ludwig.) 



surface ; the middle one is the rudiment of the tentacle, the 

 lateral processes are the first two pairs of tube-feet. At the same 

 time five elevations of the opposite wall become evident, and give 

 rise to the beginnings of the dorsal regions of the arms (Fig. 318). 

 The transition from the larval stage to the condition of the five- 

 rayed Starfish (Fig. 317) is effected by the abortion of the pre- 

 oral lobe (which, on the larva becoming fixed by means of the 

 sucker, degenerates into a temporary stalk and eventually becomes 

 completely absorbed) by the further development of the arms and 

 tube-feet, and by certain changes which take place in the internal 

 organs. Of these, one of the most important is the formation of 

 a new mouth and oesophagus (Fig. 315, B, ces), the larval mouth and 

 oesophagus becoming abolished during the metamorphosis. Round 

 this new mouth grows the ring- vessel of the ambulacral system. 

 From the stomach, diverticula grow out radially into the developing 

 arms to give rise to the caeca : and later the permanent anal 

 opening is formed on the dorsal surface. 



