CHARACTERISTIC DEEP-SEA TYPES. SEA-URCHINS. 



91 



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

 bined with an arrangement of tubercles and o,f peculiar spines 

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



Fig. 353. — Salenia Pattersoni 



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 arrange- 



Fig\ 354. — Salenia varispina. 



