In Echinoneus, the shell is described by M.

Perrier as bearing two kinds of pedicellariae, one resembling those of Echinus, and the other those of Spatangus; and such cases are always interesting as affording the means of apparently sudden transitions, through the abortion of one of the two states of an organ.

With respect to the steps by which these curious organs have been evolved, Mr.

Agassiz infers from his own researches and those of Mr.

Muller, that both in star-fishes and sea-urchins the pedicellariae must undoubtedly be looked at as modified spines.