Loc. cit.
See note on p. [541].
"Die Wirkung von Licht und Schatten auf die Seeigel," Zeitschr. für Biol. xl. 1900, p. 447.
In the aberrant genus Asthenosoma, where there are internal radial muscles, there is also an internal series of nerve-cells on the radial cord.
We prefer the term "compasses," to avoid confusion with the other meanings of the word "radius."