THE ECHINODERMS. ASYMMETRY. 



429 



retained to any marked extent as a permanent feature of the adult. It is, how- 

 ever, a familiar occurrence in the bopeiridae and paguridae, although in the last 

 two cases it affects only the terminal metameres, producing various degrees of 

 curvature, but in no wise disguising their morphological characters. 



In Limulus, a considerable number of half embryos are always present in 

 material that has been produced and developed under apparently normal condi- 

 tions; such embroyos probably occur in other arthropods more frequently than 

 we have supposed. The half embryos, in their readjustment to the new con- 

 ditions of growth, inevitably take on a bow-shaped, spiral, or semicircular form, 

 and may live for several months, although I have never known them to develop 

 beyond the trilobite stage. 



FIG. 295. Diagrams illustrating the development and metamorphosis of a crinoid (Antedon). C, Plan of an 

 early stage seen from the neural surface in mercator projection; D, E, F, G, larvae seen from the side, as semi- 

 transparent objects, illustrating the mode of fixation, the revolution, formation of the atrial chamber and of the 

 cephalic stalk. 



We assume that half embryos of this nature occurred frequently in the ar- 

 thropod ancestors of the echinoderms; that they were capable of an organic re- 

 adjustment that enabled them to survive, and that they became the prevailing 

 form. Whatever animals are assumed to be the ancestors of the echinoderms, 

 it is obvious that half embryos have been produced by them, they have survived, 

 and they have given rise to a new class of animals. 



It is a significant fact for the student of creative evolution that the absence 

 of growth on one side of an originally bilateral animal, whatever may have been 

 the cause of that, inevitably compels the remaining side to assume a new form, 

 and thus creates, at almost a single stroke, a new class of animals. In other 



