XVI 



ECHINODEEMATA 



465 



dorsal arms (jiAm, Fig. 358). Where the sides of the post-oral 

 band pass into the anal loop two long arms are developed, termed 

 the postero-lateral arms {p.l.a, Fig. 358). Finally, from the sides of 

 the anal loop two short arms are developed, termed the post-oral 

 arms (^J.o.a, Fig. 358). 



The two coelomic sacs are, for a considerable time, two somewhat 

 rounded pockets lying at the sides of the oesophagus. The one on 



a:Ci 



mt 



Fig. 368. — Fully developed Bipiuuaria larva of Asterias vulgaris, atout three weeks old. 



(After Field.) 



A, from the ventral surface. B, from the dor.sal surface, a, anus ; a.d.a, anterior dorsal arm ; cil.ad, 

 adoral ciliated band ; fus, anterior fusion of right and left coelomic sac ; int, intestine ; l.coe, left coelomic 

 sac ; m.d.a, median dorsal arm ; m.p, madreporic pore ; oes, oesophagus ; p.d.a, postero-dorsal arm ; p.l.a, 

 postero-lateral arm ; p.o.a, post-oral arm ; p.o.b, post-oral band of cilia ; pr.o.a, prae-oral arm ; pr.o.h, 

 prae-oral band of cilia ; r.coe, right coelomic sac ; at, stomach ; stom, stomodaeum. 



the left side sends up a short vertical outgrowth which fuses with a 

 slight inward dip of the ectoderm, and in this way forms a canal 

 leading to the exterior. The opening of this canal is termed the 

 primary madreporic pore (m-p, Fig. 359), and the canal itself is 

 termed the pore-canal. Its wall is covered with cilia which beat 

 inwards and strive to distend the coelomic sac with sea-water. 



Twenty years ago Field (1894) stated that, in the case of Asterias 

 vulgaris, the right coelomic sac formed a similar pore-canal, terminat- 

 ing in a right madreporic pore (m^^i. Fig. 359), which soon, how- 



VOL. I 2 H 



