92 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



fluid, the perivisceral fluid, which bathes all of the tissues and 

 fills the body-cavity. 



The perivisceral fluid is aerated through the respiratory caeca 

 which, as we have seen, are outpocketings of the thin body- 

 wall which extend outward between the calcareous plates of 

 the body. Surrounding the stone canal is a thin membranous 

 tube, and within and by the side of the stone canal is a soft 

 tubular sac. The functions of these organs are not certainly 

 known. 



calcareous spine 



respiratory caeca 



ossicles 



ampulla 



'tube fool 



epithelium of (he body 



cavity y$ 



""^r L?ft 



mesentery 

 pyloric caecumr 



ambulacral ossicle- 

 ectodermal covering 



pedicellaria ^H/w ,/ 



.radial' canal 

 radial blood-vessel 



FIG. 32. Semi-diagrammatic figure of cross-section of the ray of a 



starfish, Astcrias sp. 



The nervous system consists of a nerve-ring about the mouth, 

 and nerves running from this ring beneath the radial canals 

 along each arm. 



The starfish feed upon other marine animals, especially on 

 molluscs. They are often very destructive on oyster beds, 

 where they may occur in such numbers as wholly to deplete 

 the beds unless they are removed by means of rakes or tangles 

 of ropes that are dragged over the beds for this purpose. 



When the starfish wishes to feed on a mollusc that is too 



