8 COMPARATIVE ANATOMY. 



the base of the tubercle, and in addition produce all the 

 motion of which these organs are capable. 



It will be noticed that in addition to the absence of any 

 lateral prolongation of the body into rays, an Echinus differs 

 from a Starfish in the enormous development of the ambulacral 

 at the expense of the ant-ambulacral region : so that the latter 

 eventually becomes reduced to a very small space round the 

 apex. As in the Starfish, the test is bilaterally symmetrical, 

 for on each side of the vertical plane in which the mouth, 

 anus, and madreporic tubercle lie, there are two and a half 

 ambulacra, and two and a half inter-ambulacra. The dis- 

 position of the ambulacra into bivium and trivium is also 

 obvious. 



11. Two Sea-Urchins (Echinus sphcera), with the spines and 

 tube-feet intact. 



The spines cover the whole of the ambulacra and ant-ambu- 

 lacra. They fail however to be developed for some little distance 

 round the mouth, though they extend up to, and even arch 

 over, the anus. Though they vary much in size, they are all of 

 one shape. Each spine is cylindriform, striated longitudinally 

 as well as transversely, and tapers gradually towards its 

 apex, which is tipped with purple. The spines that form a 

 continuous series from one end to the other of an ambulacrum 

 or inter-ambulacrum are distinguished as 'primary' spines, while 

 the smaller and less regularly arranged are, in proportion to 

 their size, termed 'secondary' or 'tertiary.' The five pairs of 

 rows of tube- feet are to be seen protruding from the pores. 

 Each foot is a tubular csecal process, terminating distally in a 

 slightly expanded, disk-like extremity: while internally it is in 

 connection with the 'ampullae' and one of the 'radial trunks' of 

 the water-vascular system. In the peristomial area, and im- 

 mediately surrounding the mouth, the tube-feet become modified 

 into a circle of more or less plumose appendages, which may 

 be regarded as the aborted rudiments of the tentacular crown 

 of the Holothuroidea. 



12. A Sea-Urchin (E. brevispinosus\ bisected horizontally, 

 to shew the internal surface of the shell. 



The ambulacral plates are produced into five short truncated 

 processes, so perforated as to form five small vertical arches, 



