EECAPITULATIOX. 57 



swells at the extremity, the walls become thinner, the pouch formed at 

 the end of this cavity develops laterally, forming two smaller pouches, 

 which afterwards become hollow bodies, entirely separated from the main 

 cavity, whence they originated (the problematic bodies of Miiller). 



Tornaiia Stage. — The main cavity bends slightly towards one side, and 

 eventually unites with a depression formed there. This depression be- 

 comes the mouth ; the other opening, which was the first to be devel- 

 oped, and served the purpose of a mouth, is changed to an anus. This 

 agrees with the observations of Krohn, who shows that in an Echinus 

 larva the mouth is formed after the anus. The bent tube, or cavity, 

 divides into three distinct regions, forming the cesophagus, the stomach, 

 and the alimentary canal. 



Brachina Stage. — The small disconnected hollow bodies (the water- 

 tubes, the problematic bodies of Miiller) are not alike ; the left one {left, 

 when seen from above) connects with the surrounding medium by means 

 of an opening, the water-pore. This opening in the Starfish is the mad- 

 reporic body. The water-tubes elongate so as to reach beyond the 

 mouth, when i\\Qy approach each other and unite, forming a Y-shaped 

 tube. 



Brachiolaria Stage. — Arms are developed from the sides of the larva, 

 edged with rows of vibratile cilia. Some of these ariiis are of a different 

 character, having peculiar appendages, the so-called brachiolar arms. It 

 is on the outer surface of the water-tubes that the Starfish is developed 

 (not from the stomach, as stated by Miiller) ; one of the tubes, the left, 

 when seen from above, developing the actinal or ambulacral side, the 

 other developing the abactinal area. These two areas are open, pentago- 

 nal, warped, spiral surfoces, making almost a right angle with each other. 

 The open pentagons do not close till after the Starfish has resorbed the 

 whole of the larva. 



Echinodermoidal Stage. — The complicated system of arms and the whole 

 of the Brachiolaria are resorbed by the Starfish, which does not separate 

 from the larval stock, as seems to be the case of Bipinnaria, from the 

 statements of Miiller and of Koren and Danielssen. The arms of the 

 Starfish are broad and short in the young, and not symmetrical; the 

 suckers are pointed, have no terminal disk, and are arranged in two rows, 

 the sucking disk being developed later. The embryo, if compared to Aca- 

 lephs, might then appropriately be said to be in its Ephyra stage. The 



