THE DEVELOPMENT OP ASTEEINA GIBBOSA. 241 



which is the axial sinus, embedded in the wall of which is 

 the stone-canal (PI. XVIII, figs. 110-118). This axial sinus 



Fig. 1. 



ph. 1.2, &e. Rudiments of the outer perihsemal ring. a'. Axial sinus and 

 its outgrowth the inner perihsqaial ring. ab. Aboral sinus, gen.r. 

 Genital rachis. 



was supposed to open at its upper end into an aboral perihsemal 

 ring or pentagon, from which in each interradius two canals 

 branched off to go to the genital organs. As is well known, 

 these spaces were called ''perihsemal" by Ludwig (10), because 

 he imagined that he had discovered the true blood-system in 

 the form of curious tracts of tissue embedded in the longitudinal 

 septa of the radial canals, and in the septum separating the 

 two circular canals. He further supposed that that curious 



