178 MORPHOLOGY OF INVERTEBRATE TYPES 



longer canal on the opposite side of the radial canal. The short 

 and long canals of each side are alternating. At the end of each 

 transverse canal is a special muscular hollow organ, consisting 

 of an ambulacra! foot with an acetabulum or sucker and an am- 

 pulla. The ampulla? are in the body cavity, while the ambulacral 

 feet protrude to the outside through the pores between the am- 

 bulacral plates. The first five pairs of ampullae sitting over the 

 ossicles in which the ring canal is inclosed, are connected with 

 oral tentacles. These tentacles are simply somewhat modified 

 ambulacral tubes. Real polian vesicles opening directly into the 

 ring canal are absent in A. forbesi, but the five pairs of ampullae 

 just mentioned are sometimes called by this name. The sea 

 water admitted to the system through the madreporic plate 

 mixes with the amcebocytes produced by the Tiedemann bodies 

 and is forced by the compression of the ampullae into the am- 

 bulacral feet w r hich become extended and serve the purpose of 

 locomotion. The suckers at the end of each foot allow perfect 

 adhesion to any surface and are made use of also when the star- 

 fish intends to open the shells of a mollusc to feed upon the ani- 

 mals. It is probable that the ambulacral system plays also the 

 role of an excretory organ. 



The circulatory system follows the course of the ambulacral 

 system under which it is situated. It is composed of a system of 

 sinuses and a system of lacuna; with the axial organ. Immedi- 

 ately under the ambulacral ring is the oral sinus ring divided by 

 a perforated septum into an external and an internal oral sinus. 

 The external sinus gives rise to five radial sinuses. Each radial 

 sinus is divided longitudinally by a septum inclosing the radial 

 lacuna. The radial sinus runs under the radial canal of the 

 ambulacral system to the end of the arm, the left and right 

 channel uniting in the tentacle. The radial sinuses give off 

 transverse sinuses to the ambulacral feet. The internal oral 

 sinus gives rise to the axial sinus which runs along the hydro- 

 phoric canal with which it stands in communication by a distal 

 orifice. Communicating with the dorsal end of the axial sinus 



