112 LABORATORY WORK. 



untouched. Now find on the aboral surface of each 

 hepatic caecum the hepatic duct. Trace these ducts in- 

 ward 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 branchial 

 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 (difficult to demonstrate in the preserved speci- 

 men). Notice the openings into the branchiae. 



Remove the hepatic caeca from one arm and find the 

 lobular reproductive organs near the base of the ray. 

 Where does this duct connect with the external wall? 

 Would you consider this point (at which the duct opens 

 to the exterior) as radial or interradial? 



Below (that is, oral to) the pylorus is the cardiac portion 

 of the stomach, produced into gastric pouches in each of 

 the rays. Trace from these pouches the thin retractor 

 muscles into the ray to their attachment to its floor. 



Make a sketch of your dissection, showing in the centre 

 the stomach, in one arm the hepatic caeca, in "a second the 

 reproductive organs, a third the cardiac retractors and 

 ampullae, a fourth showing the dorsal surface, and leave 

 the other arm for structures to be added later. 



Carefully cut away stomach a little inside the mouth, 

 and then trace the stone-canal (a hard S-shaped tube) 

 downward from the madreporite to the region around the 

 mouth, Examine this circumoral region from the aboral 



