WATER SYSTEM. 



Agassiz first gave an accurate account of the water system of Echino- 

 derms. His discovery of the stone canal, and its connection with the circular 

 actinal water ring and with the ambulacral canals, as well as his subsequent 

 discovery of the connection of this system with the circulatory system, have 

 laid the foundation of the present knowledge of the water system. 



The stone canal, starting from the madreporic body, runs along the heart 

 to the circular ring. Its abactinal extremity is protected by a funnel-like 

 prolongation of the madreporic genital plate, which is very prominent in all 

 Petalosticha (PL XIV. f. i-.i, 7; PL XV. f. in; PI. XXXII. f. w ; PI. 

 XXXIV. f. 1, 8), though also present in the Desmosticha and Clypeastroids, 

 but not usually so fully developed (PL XXVII f. 2). The circular actinal 

 ring surrounds the oesophagus close to the lantern in the Desmosticha and 

 Clypeastroids ; with this are connected the five Polian vesicles, and from it 

 also branch the five main radiating ambulacral tubes, which in the Desmos- 

 ticha and Clypeastroids run towards the auricles, pass through them, and ex- 

 tend along the median ambulacral line to the abactinal pole. These tubes 

 are quite large near the circular actinal ring, gradually diminishing in size 

 towards the abactinal pole (PI. XXX. lower fig.). There are no Polian vesicles 

 in either the Clypeastroids or the Petalosticha, and in the latter the circular 

 actinal pentagon runs immediately round the actinostome, close to the level of 

 the actinal floor (PI. XXXII f. 5, 11 ; PI XXXIII f. 1 ; PI XXXIV f. 2, 4)- 



From the main central ambulacral tube, which is lined with vibratile cilia, 

 branches are sent off at right angles, on both sides ; one branch to each pair 

 of pores, each branch forming a vesicle for each pore, and the two vesicles 

 connecting with a single exterior ambulacral tentacle. These tentacles are 

 expanded by the contraction of the vesicles, and when drawn in the vesicles 

 are fully expanded. In the Desmosticha we distinguish usually the shaft 

 and the sucking-disk (PL X. f. 5, 6). The shaft is provided with longitu- 

 dinal muscles, and is strengthened by numerous spicules ; at the base of 

 the sucking-disk the spicules form a regular calcareous ring surmounted 

 by polygonal plates, forming the so-called rosette. This rosette is very 



