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 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. ^^-. 



