XVIII 



SPATANGOIDEA 



551 



The sphaeridia are situated in open pits, one or two in each, 

 situated at the bases of the tube-feet nearest the mouth. 



When the upper part of the test is picked away, the course of 

 the alimentary canal is exposed (Fig. 247). It is very similar 

 to the alimentary canal of Echinarachnius, except that from the 

 first coil a large blind pouch, called the caecum, is given off. 



The water-vascular system shows many characteristic features. 

 The tube-feet are confined to two rows in each ambulacrum, the 

 scattered smaller feet found in such abundance in Echinarachnius 

 being entirely absent. There are four distinct varieties of tube- 

 feet in Echinocarclium, which are as follows : (a) The respiratory 

 tube-feet of the petals. These have, as in Echinarachnius, broad 



plastron 



petals-: 



mouth 



modified 

 petal 



Fig. 245. 



Echinocardium cordahim. A, aboral vjew ; B, oral view, x 1. 



flat bases, but they have lost the sucker. (b) The prehensile 

 tube-feet of the anterior ambulacrum. These are enormously 

 long structures, measuring when expanded several times the 

 length of the body. They end in discs, which are frayed out 

 into fingers, so as to look like miniature sea-anemones. These 

 tube-feet are comparatively few in number and are confined to 

 the apical portion of the anterior ambulacrum. (c) The buccal 

 tube-feet. These are short, thick, and pointed, and covered with 

 a multitude of club-shaped processes. They are found on all 

 the ambulacra in the neighbourhood of the mouth. (d) The 

 degenerate tube-feet found in the portions of the ambulacra between 

 the " floscelle " (see p. 553) and the petals. These are single and 

 pointed, few in number, and issue from single pores in the test. 

 This extraordinary diversity in the tube-feet is fully explained 



