XVI ASTERIAS WATER-VASCULAR SYSTEM 44 I 



When the meal is finished the stomach is restored to its 

 former place by the action of five pairs of retractor muscles, one 

 pair of which originates from the upper surface of the ambulacral 

 ossicles in each arm and extends to the wall of the stomach, 

 where they are inserted (Fig. 190, ret). 



The tube-feet, which are at once the locomotor and the prin- 

 cipal sensory organs of the Starfish, are appendages of that peculiar 

 system of tubes known as the water-vascular system, which is 

 derived from a part of the coelom cut olf from the rest during 

 the development of the animal. This system, as already men- 

 tioned, consists of (1) a narrow " ring-canal," encircling the mouth 

 and lying on the inner surface of the membranous peristome ; 



(2) a radial canal leaving the ring-canal and running along the 

 under surface of each arm just above the ambulacral groove ; 



(3) a vertical stone-canal running from the madreporite down- 

 wards to open into the ring-canal in the interspace between two 

 arms. The madreporite is covered externally by grooves lined with 

 long cilia, and is pierced with narrow canals of excessively fine 

 calibre, the walls of which are also lined by powerful cilia. Most of 

 these narrow canals open below into a main collecting canal, the 

 stone-canal, but some open into a division of the coelom termed 

 the axial sinus, with which also the stone-canal communicates by a 

 lateral opening. The cavity of the stone-canal is reduced by the 

 outgrowth from its walls of a peculiar Y-shaped projection, the 

 ends being rolled on themselves in a complicated way (Fig. 190, B). 

 The walls of the canal consist of a layer of very long narrow cells, 

 which carry powerful flagella, and outside this of a crust of 

 calcareous deposit, which gives rigidity to the walls and has 

 suggested the name stone-canal. 



The tube-feet are covered externally by ectoderm, inside which 

 is a tube in connexion with the radial water-vascular canal. This 

 latter is lined by flattened cells, which in the very young Star- 

 fish are prolonged into muscular tails ; in the older animal these 

 tails are separated off as a distinct muscular layer lying between 

 the ectoderm and the cells lining the cavity of the tube. The 

 tube-foot is prolonged inwards into a bulb termed the " ampulla," 

 which projects into the coelom of the arm and in consequence is 

 covered outside by somatic peritoneum. Just where the ampulla 

 passes into the tube -foot proper the organ passes downwards 

 between two of the powerful ambulacral ossicles which support 



