DISSECTION OF STARFISH. 279 



surface are numbers of fleshy, finger-like projections, the 

 branchiae. Look at the very tip of the arm, and find the 

 rounded red eye- spot (recognized with difficulty in pre- 

 served material). 



INTEBKAL STKUCTUKE. 



Cut into the side of one of the arms, carrying the inci- 

 sion outward to near the tip, crossing to the opposite side 

 and then back towards but not quite to the disc. Fold 

 back the flap thus separated, and notice the following struc- 

 tures : 



Attached to the aboral surface the lobular hepatic caeca, 

 each supported by a thin membrane (mesentery). 



On the floor (oral surface) a series of thin- walled vesicles, 

 the ampullae. By means of a needle ascertain if these am- 

 pullae are connected with the ambulacra. 



Continue the removal of the aboral surface from the rest 

 of the body, taking care that all soft parts are separated 

 from it and left in the oral portion, and that the portion 

 immediately around the madreporite be left intact, and that 

 one arm be left untouched. Now find on the aboral sur- 

 face of each hepatic caecum the hepatic duct. Trace these 

 ducts inward until they enter a saccular structure, the 

 pyloric part of the stomach. Do they unite before joining 

 the stomach ? On the aboral surface of the pylorus is a 

 small lobular structure, the branchial tree. How many 

 branches has it ? Is it radial or interradial in position ? 

 Draw a line through the starfish passing through the bran- 

 chial tree, dividing the animal into symmetrical halves ; how 

 does this symmetry compare with that obtained from the 

 madreporite ? Near the centre of the pylorus is the small 

 tubular intestine (frequently torn in removing the external 

 wall). It empties by a vent on the centre of the disc; diffi- 



