510 



INVERTEBRATA 



CHAP. 



arms. These arms are capable of movement, and can be slowly approxi- 

 mated or divaricated. These movements are due to muscular strands 

 connecting the rods near the aborar- pole, and to others going .out 

 to them from the ends of the oesophagus. Both sets are derived 

 from cells of the secondary mesenchyme. 



When it is about twelve days old the left anterior coelom buds off a 



mp 



FIG. 386. Echinopluteus larva of Echinus esculentus about eleven clays, viewed from the 

 dorsal surface to show the formation of the ciliated epaulettes. (Original.) 



Names as in previous figure. In addition, a.cil.ep, anterior ciliated epaulette ; am, amniotic in- 

 vagination ; d.a, dorsal arch ; hy, hydrocoele ; La.c, left anterior coelom ; l.p.c, left posterior coelom ; 

 m.p, madreporic pore; m.v, madreporic vesicle; p.c, pore-canal; p.d.a, postero-dorsal arm; p.d.r, 

 postero-dorsal rod ; pr.v.a, prae-oral arm ; r.a.c, right anterior coelom ; r.p.c, right posterior coelom ; 

 st.c, stone-canal. 



posterior vesicle, which remains connected with it by a narrow neck. 

 The vesicle is the hydrocoele which gives rise to the adult water- 

 vascular system, and the neck is the stone-canal. Where this neck 

 joins the anterior coelom the latter is dilated, and this dilatation is 

 the rudiment of the axial sinus of the adult. 



At about sixteen days the right anterior coelom buds off a solid 

 mass of cells which, for a short time, remains connected with it by a 



