ECHINOIDEA. 233 



supported by calcareous pieces, which form a ring round the margin of 

 the sucker (Fig. 169). In many forms the abactinal tube-feet are pointed 

 and are without the calcareous ring, and in all cases the suckers of the 

 abactinal feet are weaker than the others. The suctorial tube-feet 

 together with the spines form the organs of locomotion. 



In the Cidaridae and Echinothuridae the feet which come through the 

 ambulacral plates of the peristomial membrane are similar to the other 

 feet. In forms with only ten perforated ambulacral plates on the peri- 

 stome, the tube-feet which they transmit, the oral tube-feet or buccal 

 tentacles, are small and terminate in an oval disc. They move actively 

 when in the neighbourhood of food, without, however, touching it. They 

 are supposed to be olfactory or gustatory in function. 



In the Clypeastroids and Spatangoids the feet of the petals differ from 

 those of the rest of the ambulacral system. They are broadened at the 

 base, and their sides are indented or sacculated ; their walls are without 

 calcareous bodies. They are said to be respiratory in function and are 

 called ambulacral gills (see p. 214). 



The tube-feet issuing from the fine pores of Clypeastroids are locomo- 

 tive. They are cylindrical in form, are provided with calcareous bodies, 

 and end in a sucker (which may be supported by a calcareous ring). 



In Spatangoids the feet are very various in shape, according to the 

 part of the body in which they are placed. There are (1) the respiratory 

 feet of the petals without terminal suckers or calcareous bodies ; (2) 

 ordinary locomotive feet with suckers and calcareous supports ; (3) 

 simple tactile feet without suckers ; (4) brush-like tactile feet found round 

 the mouth and the anus and in the Cassidulidae on the phyllodes ; they 

 terminate in an expanded disc which carries a number of club-shaped 

 'filaments, each of which is supported by a calcareous rod ; (5) the rosette- 

 feet of the anterior ambulacrum ; these end in discs the edges of which 

 are drawn out into short processes supported by calcareous rods ; they 

 are often of great length and are prehensile in function, seizing food which 

 is to be conveyed to the mouth. The feet of the petals are in connexion 

 with double pores in the shell, the other feet only having one pore. 



According to J. Miiller the locomotive tube-feet of the Spatangoids are 

 less numerous, while those of Clypeastroids are far more numerous, than in 

 regular Echinoids. 



The vascular system attains a development somewhat similar 

 to that which it has in Holothurians. It consists of a plexus in 

 the intestinal wall connected with two longitudinal intestinal 

 trunks, which lie in the mesenteries. These open into a cir- 

 cumoral vessel * from which pass five radial vessels. The latter 

 lie between the water- vascular canals and the radial perihaemal 

 space, and give off vessels to the tube-feet. The circumoral 

 blood-vessel is close to the circumoral water-vascular trunk. 

 As in other cases the significance of this system is obscure : it 

 consists largely of lacunar tissue. 



* The word vessel is not perhaps correctly applied to the various tracts 

 and branches of this lacunar tissue. 



