172 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Each crustacean limb typically consists of a basal piece, the protopodite, 

 with two jointed branches rising from it, the internal endopodite and the external 

 exopodite, though in many forms the outer branch disappears; the protopodite has 

 usually two segments, a basal or proximal coxopodite and a distal basipodite; it 

 is the specialization of certain of the appendages to function as masticating organs 

 which especially characterizes arthropods as compared with annelids. The struc- 

 ture of the highly complicated "Aristotle's lantern" in the echinoids, and of the 

 equally complicated arms of the crinoids, is reducible to the structure of the primi- 

 tive crustacean appendage, plus the internal accessory structures, while the speciali- 

 zation of certain of the appendages to function as masticating organs, or at least as 

 mouth plates, is as characteristic of the echinoderms as it is of the crustaceans. 



The mandibles in the articulates are such highly specialized appendages and 

 so intimately connected with the digestive tube that on reflection it is not sur- 

 prising to find them in a modified form carried over into the echinoderms. 



The mandibular structures in the holothurians are very rudimentary, and this 

 set passes backward without attaining any further perfection. No sooner does 

 one set of mandibular structures pass backward from a position about the mouth 

 than another immediately forms there to take its place. 



This second set in the echinoids has attained a most extraordinary development. 



In the crinoids, in which tliis also has moved backward and lost its great com- 

 plexity, though retaining in the long and tapering arms an extraordinary number 

 of individual elements. 



The third set, the crinoid orals, developed about the mouth on account of the 

 moving away of the second set to form the arms, are very large, but extremely 

 simple in structure, and often become entirely resorbed in fully grown animals, 

 though when this is the case a fourth set sometimes replaces them. 



The interpretation of the free undivided arms of the crinoids as remotely 

 homologous to arthropod appendages explains how the ambulacral grooves and 

 other ambulacral structures happen to be drawn out upon them. Intimately con- 

 nected with the mouth, upon moving away from it they drag with them much of 

 the circumoral structures. 



In the same way, in migrating backward over the surface the coronal ring of 

 the echinoids has carried with it extensions from certain of the mouth structures, 

 as, for instance, the water vascular tubes. 



Embryology teaches us that there is a constant and well-defined path followed 

 by developing structures. All developmental processes first begin at the head end 

 of the embryo and gradually extend backward toward the tail. 



In the echinoderrns the longitudinal axis of the bilateral invertebrate is resolved 

 into a circle and a straight line passing through the center of the circle, the circle 

 representing the axis of the somites, the straight line that of the digestive tube. 



The circle, with no beginning and no end, has ceased to function as a true 

 axis, or to have any other significance, leaving the line at right angles to its plane 

 as the only functional axis. 



