ANNULOIDA : ECHINODERMATA. I47 



these consists of a stem, bearing two or three blades or claws, 

 which snap together and close upon foreign objects, like the 

 beak of a bird. Their action appears to be independent of 

 the will of the animal, and their true function is not known ; 

 but they may be regarded as peculiarly modified spines. 



Locomotion in the Echinoidea is effected by means of a 

 singular system of contractile and retractile tubes, which con- 

 stitute the " ambulacral tubes," or " tube-feet," and are con- 

 nected with the " ambulacral system " of aquiferous canals (fig. 

 41). 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 

 conveyed 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 

 certain vesicles of unknown function, known as the " Polian 

 vesicles " (ampullce PoliancB). 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 fol- 

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

 secondary lateral branch, which terminates in a vesicle. These 

 vesicles or "ampullae" 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 

 ii; 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- 

 8 



