384 



ZOOLOGY 



SECT. 





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Fio. 321. Larva of Asterina, view 

 of the left side, showing the five- 

 lobed prominence (amb.) formed by 

 the developing ambulacral system 

 on what is destined to become the 

 ventral surface of the body of the 

 Starfish ; larv. org., pre-oral lote 

 with larval organ. 



tent 



coele, grows into the form of a horse-shoe with five lobes, each of 

 which represents one of thejradial 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 hydro - 

 ccele 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. 

 321, amb), each of the lobes corre- 

 sponding to one of the five lobes of the 

 hydrocoele. Each of the; latter then becomes divided, first into 

 three rounded processes (Fig. 320, B, amb), and then into five, 

 and these project freely 

 on the surface ; the 

 middle one is the rudi- 

 ment of the tentacle, 

 the lateral processes are 

 the first two pairs of 

 tube-feet. At the same 

 time five elevations of 

 the opposite wall be- 

 come evident, and give 

 rise to the beginnings 

 of the dorsal regions of 

 the arms (Fig. 323). 



The transition from 

 the larval stage to the 

 condition of the five- 

 rayed Starfish (Fig. 322) 

 is effected by the abor- 

 tion 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 cert.ain changes which take place in the internal 

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



FIG 322. Asterina exigua. Young Starfish shortly 

 after the metamorphosis has been completed, viewed 

 from the oral side. circ. amb. circular ambulacral 

 vessel ; dors. p. dorsal pore and madreporic canal ; rad. 

 amb, radial ambulacral vessel ; st. stomach ; tent . 

 tentacle ; t. f. tube-feet. 



