BODY-WALL AND CCELOM 289 



communicates with the ectodermal system and consists of motor 

 fibres for the muscles of the large ossicles and body-wall. Thus 

 the nervous system is in a more primitive condition than that 

 of any other animals except the coelenteratcs. Below the epi- 

 thelium, the body-wall is composed of connective tissue, in which 

 the ossicles are embedded. The deeper part contains some 

 muscular fibres running in various directions. Within the body- 

 wall there is a spacious cavity, the perivisceral coelom, which 

 extends into the arms, and contains the alimentary canal and 



pb.s^ 9-)- 



py.d 



r.iv.v 



n.r \ 



e.ph 



i.ph / 

 w.v.r perist 



i-ph',r.ph r.n 

 bv r 





Fig. 215. — Diagram of a section of a starfish passing through the madreporic 

 interradius and along the opposite arm, a Uttle to one side of the septum 

 of the radial perihaemal vessel. 



an.. Anus; ax.o., axial organ; ax.s., axial sinus; b.v.r., so-called blood vascular ring ;<-/>., epidermis; 

 g., gill ; g.r., genital rachis, in an aboral coelomic ring sinus ; i.ph., inner ' perihaemal ' ring ; w., mouth; 

 mad madreporite ; muse, one of the muscles that narrow the ambulacral groove ; n.r., nerve rmg; 

 e.ph., outer perihsmal ring ; oss., ossicles ; p.c, perivisceral cavity ; pb.s., peribranchial smus; 

 perist., peristome ; py.c, pvloric caecum ; py.d., pyloric duct ; py.s., pyloric sac ; r.c, rectal caecum ; 

 rm. rectum; r.n., radial nerve; r.ph., radial perihaemal vessel; r.aw., radial water vessel; sep., 

 septum ; sp., spine ; 5^., stomach ; st.c, stone canal ; s.t., sense tentacle ; uw.r., water vascular rmg. 



generative organs. The coelom is lined by a ciliated peritoneal 

 epithelium, and along the oral side of each arm, where the body- 

 wall roofs the ambulacral groove, runs a ridge, the ambulacral 

 ridge, caused by the projection of a double row of large, trans- 

 versely placed ambulacral ossicles. At the outer ends of these, 

 alternating with them, lie smaller adambulacral ossicles, and upon 

 the adambulacral ossicles stand the adambulacral spines. In each 

 interradius a stiff interradial septum projects into the coelom 

 between the arms. To the septum which is situated in the inter- 

 radius of the madreporite there is attached a sac, the axial sinus, 

 also a part of the ccelom, and in this are lodged the stone canal, 

 which, as will presently be stated, runs downwards from the 



