CHARACTERISTIC DEEP-SEA TYPES. -- SEA-URCHINS. 91 



suranal plate of an asymmetrical apical system (Fig. 353), com- 

 bined with an arrangement of tubercles and of peculiar spines 

 which connect it on the one side with the Cidaridae, and on the 



Fig. 353. Salenia Patterson!. 



other with the more recent types of sea-urchins. This asymme- 

 try is an embryonic character of echinoderms, due to the spiral 

 disposition of the plates of the embryo. Traces of this arrauge- 



Fig. 354. Salenia varispina. 



