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 Cidaridee, and on the 
Fig. 353. — Salenia Pattersoni. 4. 
other with the more recent types of sea-urchins. This asymme- 
try is an embryonie character of echinoderms, due to the spiral 
disposition of the plates of the embryo. Traces of this arrange- 
Fig. 354. — Salenia varispina. 99. 
