3IO THE STARFISH less. 



The circular blood-vessel (C B. V) surrounds the gullet and gives 

 off radial vessels {Nad. B. V) to the arms and an inter-radial plexus 

 connected with a pentagonal ring round the intestine. 



The circular ambulacral vessel (C. Amb. V) gives off radial vessels 

 {Kad. Amb. V) to the arms connected with the ampulla; {Amp) and 

 tube feet {T. F): it is also connected with the stone-canal {St. C), which 

 opens externally by the madreporite {Mdpr). 



The nerve-ring {Nv. R) gives off radial nerves {Rod. Nv) to the 

 arms. 



The ovary (Ovy) is inter-radial, and opens by a dorsal oviduct {Ovd). 



and is probably an organ of smell. At the base of the 

 tentacle is a bright red eye-spot {oc). 



Sections show that there is a well-marked coelome, 

 separating the body-wall from the enteric canal and contain- 

 ing the gonads, blood-vessels, &c. The body-wall consists 

 externally of a very thin cuticle, then of a layer of dene 

 epithelium or epidermis {Der. Epthm), then of a thick, 

 double, filirous layer {Derm), then of a thin and interrupted 

 layer of muscle, and finally, of a layer of coelomic epithelium 

 {Ccel. Epthtti) bounding the body-cavity. 



The ossicles with their spines together form an external 

 skeleton or exoskeleton : as already mentioned they are, for 

 the most part, small irregular bodies developed in the 

 fibrous layer of the body-wall, and overlapping one another 

 in a scale-like fashion. But the ambulacral grooves are 

 bounded by regularly arranged pairs of large, rod-like ambu- 

 lacral ossicles {Amb. os), arranged like rafters, the dorsal 

 ends of each pair uniting at the summit of the groove, 

 while their ventral ends diverge and are connected with the 

 ordinary ossicles at the edge of the arm. Between each 

 ambulacral ossicle and its predecessor and successor in the 

 row is an aperture, the ambulacral pore, with which one of 

 the tube-feet is connected. 



The mouth (Fig. 76, a, mth) leads by a short gullet into 

 z. stomach {si) divisible into two portions, called respectively 



