AMONG THE SEA-URCHINS. 



15 



even if we examine a fossil Echinoid from the antique Ordovician 

 period. The spines, when cut in thin sections, form beautiful 

 and well-known polariscopic mounts, the spine of each species 

 being formed on a different design. One of the thickest spines 

 is that from Cidaris papillata ; a segment from a slice of which is 

 shown at Plate I., Fig. 1 1. 



Fig. 2. Masticatory Apparatus of Echinus. 



We must now deal with the other principal organs of locomo- 

 tion in the Sea-Urchin, namely, the " ambulacra," or tube feet, 

 whereby this round ball is enabled to mount the sides of a 

 sub-marine precipice, or traverse the frond of a moving sea-weed. 

 These are the long waving tubes which are so noticeable on a star 

 fish or Echinus when out of the water. One of these ambulacral 

 suckers is connected with each pair of tiny holes seen in woodcut 

 (Fig. I, B), through which every tube communicates with the 

 interior of the shell. Within the Echinus there is a marvellously 

 interesting water vascular system, which ramifies throughout its 

 inner circumference, being connected with the madreporic filter (see 

 woodcut, Fig. I, C, w), from which a plentiful supply of water is 

 available at all times, being conveyed to a circular tube round 

 the mouth, (Fig. 2, C,/), from whence proceed radiating canals along 



