ARBACIA PUNCTULATA 197 



aphragm surrounding the oesophagus. All lantern muscles are 

 composed of smooth fibres. 



The entire lantern with its muscles is inclosed in the oral sinus 

 formed by the so-called peripharyngeal membrane which is simply 

 a part of the peritoneum. 



The oesophagus is a rather long tube opening into the broad and 

 flat stomach which has numerous folds or pouches. The stomach 

 makes an almost complete circle and leads into the intestine 

 which also forms an almost complete circle, running however 

 above the former in the opposite direction. The short rectum 

 runs directly upward and terminates in an anus in the centre 

 of the aboral surface of the animal. Closely applied to the inner 

 edge of the stomach but not otherwise connected with it runs the 

 siphon. It is a thin tube starting from the oesophagus not far 

 from its proximal end and opening into the intestine close to 

 the stomach. The siphon has practically the same histologic 

 structure as the stomach. The entire alimentary canal has a 

 covering of ciliated peritoneum and is suspended by mesen- 

 teries. 



The ambulacral or water vascular system. We have seen 

 already that one of the genital plates serves at the same time 

 as a madreporic plate. The hydrophone canal runs down to the 

 diaphragm where it opens into the circular canal surrounding 

 the oesophagus. The circular canal gives off five radial canals. 

 Into the circular canal, between the radial canals, open five 

 small sponge-like organs, erroneously called either Polian or 

 Tiedemann's vesicles, but histologically different from both. 

 The radial canals run between the two auricles of the same 

 radius and terminate in the terminal tentacle. They give 

 off transverse canals connected with the ambulacral tubes and 

 ampulla. The first pair of transverse canals belongs to the 

 oral papillae. Each ambulacral foot or tentacle, bifurcated at 

 its base, communicates with its ampulla by means of two open- 

 ings. This is the reason why there are two pores in the am- 

 bulacral plates for each ambulacral tentacle or foot. The am- 



