l6o MANUAL OF ZOOLOGY. 



or pentagonal plates, known as the " ocular plates," each 

 being perforated by a pore for the reception of an " ocellus " 

 or " eye." 



Besides the spines, which are sometimes of a very great 

 length, the test often bears curious little appendages, called 

 " pedicellariae," and often supposed to be parasitic. Each of 

 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. One 

 function performed by the pedicellarise, in some species at 

 any rate, is the removal of excrementitions particles of food. 

 Such particles, on being ejected from the vent, are seized by 

 the pedicellarise, passed on from one to another, and ultimately 

 entirely got rid of. 



Locomotion in the Echinoidea is effected by means of a 

 singular system of contractile and retractile tubes, which 

 constitute the " ambulacral tubes," or " tube-feet," and are 

 connected with the "ambulacral system" of aquiferous canals 

 (% 57)- 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 madre- 

 poriform 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 ambulacral 

 canals, there are connected with the circular canal certain 

 vesicles of unknown functions, known as the " Polian vesicles " 

 (ampulla Poliancz). 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 



