DEVELOPiMENT OF HOLOTHURIANS 



615 



luouth is (III llic k't't side of the larva, and in the connnencement 

 of the metamorphosis the mouth migrates into tliis position (Fig. 

 286, C). Tlien tlie rudimentary prae-oral lobe is rapidly 

 absorbed, so tliat tlie mouth again acquires a terminal posi- 

 tion. The hydrocoel (Fig. 286, A, liy) lias by this time com- 

 pletely encircled the oesopliagus, and from it grow out the radial 

 canals which bud off the feelers ^ (buccal tentacles) into the 

 larval stomodaeum. This, although it later flattens out to form 

 the adult peristome, forms in these 

 stages an almost closed sac, re- 

 minding us of the amniotic cavity 

 in the Echinopluteus. The ciliated 

 band breaks up into a number of 

 pieces, which rearrange themselves 

 so as to form a series of transverse 

 rings of cilia ; so that the free- 

 swimming life can be carried on 

 somewhat longer. The animal in 

 this stage is called a " pupa " (Fig. 

 292) ; it eventually loses the rings, 

 drops to the bottom, and develops 

 tube-feet. From specimens which 

 the author has seen, he has 

 little doubt that in some cases the 

 young animal passes through an 

 " Echinoid " stage, for it possesses, 

 besides the feelers, only median 

 tube-feet, terminating the radial canals, and it is covered l)y a 

 cuirass of plates, which recalls the Echinoid corona." 



Reviewing the development of the Eleutherozoa in the light 

 of the facts so far presented, and using the same method of 

 reasoning which is employed in the case of other groups of 

 animals, we seem to be justified in concluding that the Echino- 

 dermata are descended from a simple free-swinnuing ancestor 

 possessing the fundamental characters of the Diplcurula. These 

 would include a longitudinal folded band of cilia as the principal 

 organ of locomotion ; a thickened plate of nervous epithelium at 



Fig. 292.— "Pupa" of Sy'na2)ta digi- 

 tata. X 50. circ.cil. Ciliated rings ; 

 OSS, calcareous ossicle ; ot, otocysts ; 

 pod, feelers ; vw.r, radial water- 

 vessel. (After Seiuon.) 



^ In tlie type figured (larva of Siinajda diijilata) the feelers are huilded nil" 

 directly from the ring-canal and alternate with the rudiments of tiie radial canal. 

 - Observed in Plymouth, 1905. 



