126 MANUAL OF ZOOLOGY. 



singular system of contractile and retractile tubes, which con- 

 stitute the * ambulacral tubes,' or ' tube-feet,' and are connected 

 with the ' ambulacral system' of aquiferous canals (fig. 33, 2). 

 From the perforated ' madreporiform tubercle ' on the largest 

 of the genital plates, there proceeds a membranous canal, 

 known as the * stone,' or ' sand canal,' whereby water is con- 

 veyed from the exterior to a circular tube, surrounding the 

 oesophagus, and constituting the centre of the water- vascular 

 or ambulacral system. The function of the madreporiform 

 tubercle appears to be that of permitting the ingress of water 

 from the exterior, but of excluding any solid particles, which 

 might be injurious. The ' circular canal,' surrounding the 

 gullet, is situated between the nervous and blood-vascular 

 rings, and gives off five branches the 'radiating canals' 

 which proceed radially along the 'ambulacral areas' in the 

 interior of the shell. In this course they give off numerous 

 short lateral tubes the 'tube-feet,' which pass through 

 the ' ambulacral pores ' to gain the exterior of the test, 

 and terminate in suctorial discs. Besides the radiating am- 

 bulacral canals, there are connected with the circular canal 

 two vesicles, of unknown function, known as the ' Polian 

 vesicles' (ampullce Poliance). The ambulacral tubes, or tube- 

 feet, can be protruded at the will of the animal, through 

 the pores which perforate the ambulacral areas, and can be 

 again retracted. By means of these locomotion is effected, the 

 tube-feet being capable of protrusion to a length greater than 

 that of the longest spines of the body. The mechanism by which 

 the tube-feet are protruded and retracted, is as follows : 

 Each tube-foot, shortly after its origin, gives rise to a 

 secondary lateral branch, which terminates in a vesicle. 

 These vesicles are provided with circular muscular fibres, by 

 the contraction of which their contained fluid is forced into 

 the tube-feet, which are thus protruded. Retraction of the 

 ambulacral tubes is effected by proper muscular fibres of their 

 own, which expel again the fluid which has been forced into 

 them by the vesicles. According to Owen, the terminal sucker 

 in each tube-foot of the Echinus is ' supported by a circle 

 of five, or sometimes four, reticulate calcareous plates, which 

 intercept a central foramen, and by a single, delicate, reticu- 

 lated, perforate plate on the proximal side of the preceding 

 group. The centre of the suctorial disc is perforated by an 

 aperture conducting to the interior of the ambulacral tube- 

 foot.' This perforation of the suctorial discs of the ambulacra, 

 though affirmed by Valentin, is denied by Miiller ; and it is 

 difficult to believe that it would not impair the functions of 

 the feet in the act of protrusion. 



