No. 515] AFFINITIES OF THE ECHIXOIDEA 



small apertures with the body-cavity, and, at one point in 

 the pseudo-haemal ring, with the axial sinus. A blood vas- 

 cular system is doubtfully present in the ophiuroids and 

 asteroids, but well developed in the echinoids and 

 crinoids. In the echinoids the axial sinus surrounding 

 the stone canal, originally a part of the coelom, is in open 

 communication with the ampulla into which the madre- 

 porite opens. This is comparable to the condition in the 

 crinoids where the madreporic pores open into the body 

 cavity more or less opposite the openings of the stone 

 canals, but is quite different from the condition found in 

 the asteroids and ophiuroids. 



Having now shown that the crinoids and the echinoids 

 are closely related, it remains to be seen how an homology 

 may be drawn between the skeletal elements of the two. 

 This is not nearly so difficult a performance as might ap- 

 pear at first sight. The primibrachs of a crinoid repre- 

 sent the first four ambulacral plates of an urchin, which 

 have slipped in between each other so as to lie in a single 

 row ; in this single row of four plates the second has dis- 

 appeared, as shown by the synarthry, 2 while the third and 

 fourth have united to form an axillary. All the plates in 

 the crinoidal post-radial series up to the third brachial of 

 the free arm represent the ambulacral series of the 

 urchin ; the long and tapering crinoid arms from the third 

 brachial onward are homologous to the auricles and 

 apophyses of the urchin, which have become turned out- 

 ward instead of inward, have become interiorly united, 

 and have increased enormously in length. The crinoid 

 stem is the central (sur-anal) plate of the urchin; origin- 

 ally free, the crinoids first became sessile through simple 

 attachment by the central dorsal plate ; this gradually in- 

 creased in thickness, becoming a thick stalk, like that of 

 Holopus ; later, owing to the increasing length, fractures 

 were developed transversely, and finally the long jointed 

 crinoid stem resulted. 



It has been urged, from their radiate structure, that 

 the echinoderms were primarily fixed ; but I can not see 



•American Naturalist, Vol. 43 (October, 1909), pp. 577-587. 



