46 EMBRYOLOGY OF THE STARFISH. Part I. 



the larva, then 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). 



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

 unites with a depression formed there. This depression becomes the mouth ; the 

 other opening, which was the first to be developed, 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 oesophagus, the stomach, 

 and the alimentary canal. 



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

 blematic 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 madreporic body. The water-tubes elongate so 

 as to reach beyond the mouth, when they 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 arms 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, pentagonal, warped, spiral surfaces, 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 Acalephs, might then appropriately be said to be in its Ephyra stage. 

 The odd terminal tentacle has an eye at its base, and no disk is ever formed at the 

 extremity of this tentacle. The abactinal surface is very arched, the spines are 

 arranged in regular rows, and the arrangement of the plates reminds us of the plates 

 of Crinoids ; the plates first formed retaining their embryonic or crinoidal character. 

 The anus opens near the edge of the disk, on the lower side ; the madreporic 

 body is situated on the edge, but moves to the abactinal area, in more advanced 



