464 CHARACTERS OF INVERTEBRATE ANIMALS 



surface upon which the animal creeps presents a sharp contrast 

 with the curved upper surface. This is also the case with some of 

 the species inhabiting much shallower water, and in these bilateral 

 forms, three of the ambulacra are turned downwards, and bear 

 tube-feet used in locomotion, while the other two face upwards, 

 and their tube-feet are modified into pointed structures useful only 

 as organs of breathing and touch. The tentacles also present 

 a considerable variety in size and shape. Some common forms 

 have the tube-feet irregularly scattered over the body instead 

 of being arranged in rows, while in yet others the tube-feet, and 

 even the radial branches of the water-vascular system, are absent 

 altogether. This is the case, for example, in the genus Synapta, 

 species of which are found in British seas. Here the translucent 

 body is worm-like, and the small tentacles are feather-shaped, a 

 further character of interest being found in the shape of the 

 calcareous plates embedded in the skin, which are in the form of 

 anchors with associated oval perforated plates (anchor plates). 

 It also possesses ten minute auditory sacs in close connection 

 with the nerve-ring. 



