DEVELOPMENT OF HOLOTHURIANS 



615 



rings 

 swimming 



mouth is on the left side of the larva, and in the commencement 

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

 286, C). Then the rudimentary prae-oral lobe is rapidly 

 absorbed, so that the mouth again acquires a terminal posi- 

 tion. The hydrocoel (Fig. 286, A, hy) has by this time com- 

 pletely encircled the oesophagus, 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 

 of cilia ; so that the free- 

 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 by a 

 cuirass of plates, which recalls the Echinoid corona.^ 



Eeviewing 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 -swimming ancestor 

 possessing the fundamental characters of the Dipleurula. These 

 would include a longitudinal folded band of cilia as the principal 

 oro-an of locomotion ; a thickened plate of nervous epithelium at 



Fig. 292. — " Pupa " of Syiw,pta digi- 

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

 OSS, calcareous ossicle ; ot, otocysts ; 

 pod, feelers ; w.v.r, radial water- 

 vessel. (After Semon. ) 



^ In the type figured (larva of Synapta digitata) the feelers are budded off 

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

 2 Observed in Plymouth, 1905. 



