THYONE BRIAREUS 205 



morphologically in the same place as the anus of the sea-urchin. 

 The entire alimentary canal is provided with a muscular sheath 

 of internal longitudinal and external circular muscles. 



Respiratory system. Two water-lungs or canals opening 

 into the cloaca and furnished with many lateral branches func- 

 tion as organs of respiration. They are attached to the body 

 wall and to the intestine by mesenteries. The left lung is be- 

 sides surrounded by a network of lacunae. 



Water-vascular or ambulacral system. The ring canal sur- 

 rounds the pharynx at its junction w r ith the oesophagus. The 

 hydrophone canal ends in a madreporic plate. It does not open 

 to the outside, but into the body cavity or ccelome, since the 

 madreporic plate lies within the latter and not on the body 

 wall, a condition similar to that in Crinoidea. (The Crinoidea 

 have, however, numerous madreporic canals.) There are 

 usually two large Polian vesicles opening into the ring canal. 

 The five radial canals connected with the ring canal run first 

 forward and give off transverse canals to the five pairs of oral 

 tentacles; then turn backward and run between the two bundles 

 of longitudinal muscles to the posterior end where they ter- 

 minate blindly. The radial canals give off throughout their 

 length transverse canals to the ambulacral feet and tentacles. 

 The ampullce are small and scattered all over the inner surface 

 of the body wall. 



The circulatory system is characterized by the greater devel- 

 opment of the lacunar system and a reduction of the system of 

 sinuses. Both axial sinus and axial lacuna are absent. Five ra- 

 dial sinuses giving off transverse branches to the ambulacral 

 tubes, extend all the way to the cloaca and terminate blindly. 

 At the anterior end they open into the circular oral sinus. The 

 circular oral lacuna gives rise to five radial lacuna and two intes- 

 tinal lacunce. The ventral intestinal lacuna runs along the intes- 

 tinal canal. The dorsal intestinal lacuna runs in the mesentery, 

 giving off a series of branches which split into a network the so- 

 called rete mirabile. This network of lacunae is connected not 



