266 



s. GOTO 



reached its limit, the w;ilJs of the cavity are appressed ag-ainst each 

 othei" and form for a time a plate of cells, which subsequently disap- 

 pears, prol)ably b}' being- drawn in into the formation of the wall of 

 the axial sinus, which immediately afterwards undergoes a, rapid en- 

 largement with the growth of the star. Compare on this point Figs. 

 27 (rt) and o9. The flattening and subsequent fusion of the walls of 

 the anterior enterocœl leave two particular areas of a circular form 

 untouched, and consequently two rings are formed, one the circular 

 water-vascular canal and the other the circular enteroccel, or the inner 

 perihœmal ring of authors. From tiie above description it is clear 

 that the axial sinus is at last drawn into the primitive dorsal mesen- 

 tery dividing the right from the left enterocœl in the larva. 



The circular enterocœl is first formed on the left side (Figs. 27 & 

 39). This is in perfect accord with the fact above mentioned that the 

 flattening of the anterior enterocœd is completed first on the same side. 

 In the figures above referred to one sees the cavitv in question 

 entirely separated from the anterior enterocœl, l)ut in some sections of 

 the same series one can observe it still connected by an ii-reaailar strin«' 

 of cells with the plate of cells formed by the fusion of its walls. In Fig. 

 38, again, it is just in the prcjcess of being constricted off from the 

 anterior enterocœl. The circular enterocod is inostl}' situated nearer 

 the centre of the disc than the water-vascular ring, and remains in per- 

 manent communication witli the axial sinus in the madreporic inter- 

 radius, where the constriction leading to its formation never takes place. 



The reader has perhaps noticed in Fig. 18 (stage F) a posterior 

 extension of the axial sinus marked sin.' ax.' In this stao-e it is almost 

 completely cut off from the axial sinus proper, but can be seen in one 

 or two sections to be connected with it by a few cells extending be- 

 tween their w^alls. It is that portion of the left anterior enterocœl 

 which has been called " dc^rsal sac " by lîury, and about wdiich we 



