180 



PHYLUM ECHINODERMATA. 



spaces are called the peribranchial cavities (Fig. 129, 15, Fig. 

 132, pbr). 



The axial sinus is a tube passing from the abactinal to the 

 actinal surface in the interradius of the madreporite (Fig. 132, 

 ax.s). It is contained in an irrterbrachial septum (Fig. 130) and 

 is lined by a ciliated epithelium. The stone-canal and axial 

 organ project into it, being attached to its inner wall, i.e. to its 

 wall next the oro-anal axis, by membranes (st.c). Orally the 



me* Mils 



st.c 



my tf&i.r 



y f "' h fl ft mv < 



-;'" ' 



,jf> 



br 



ip.c. 



FIG. 132. Longitudinal vertical section through the disc and one arm of a young starfish 

 (after an original drawing by Prof. E. W. MacBride, F.R.S.). The section passes through 

 the interradius of the stone-canal and madreporite. ab aboral sinus ; ax.s axial sinus ; 

 az.t terminal unpaired tentacle ; br papula (dermal gill) ; ext.p.r external perihaemal ring- 

 canal ; gen.r genital rachis ; gen.st. genital stolon (prolongation of rachis into axial organ) ; 

 int.p.r internal perihaemal ring-canal; l.p.c left posterior coelom of larva, hypogastric 

 coelom of adult ; mes remains of the mesentery separating the left posterior (hypogastric) 

 and right posterior (epigastric) coeloms ; mp madreporic pores ; mv madreporic vesicle 

 (right hydrocoel) ; nr nerve ring ; oc eye ; pbr peribrauchial space ; per.rad.c radial peri- 

 haemal canal ; p.or.c perioral coelom, an outgrowth of the hypogastric coelom, from its 

 walls are formed the retractors of the stomach ; pyl pyloric sac ; pyl' the opening of a 

 pyloric caecum ; rail. n radial nerve ; reel rectum ; ret retractor of the stomach ; r.p.c right 

 posterior coelom (epigastric of the adult) ; st.c stone-canal ; stow, stomach ; tf tube-foot ; 

 w.v.rad.c water-vascular radial canal ; w.v. ring water-vascular ring canal. 



axial sinus is continuous with the so-called inner perihaemal 

 space which surrounds the mouth (Fig. 131, a l , Fig. 132, int.pr). 

 At its aboral end it communicates with the stone-canal on the 

 one hand and with the exterior through some of the pores of 

 the madreporite on the other (Fig. 132, mp.). The axial sinus is 

 derived from the anterior body-cavity of the larva. 



The outer circumoral perihaemal ring, which is separated by 

 a slightly oblique septum from the inner perihaemal ring just 

 described, is not really a ring at all. It is (Fig. 131, ph. I. II. etc.) 



