ECHINODERMS 453 



five-angled pyloric sac, and this into a short thin- walled intestine. 

 The cavity of each arm is largely occupied by a pair of large 

 saccular tubes which open into one of the corners of the pyloric 

 sac, and secrete a digestive juice. The circulatory organs 

 chiefly consist of an ill-developed blood-system devoid of heart, 

 and of a spacious body-cavity. Special respiratory organs are 

 evidently necessary in a creature furnished with a firm exo- 

 skeleton, and in this case the function is discharged in part by 

 delicate branched gills which can be protruded through the 

 interstices of the calcareous net- work which strengthens the upper 



CAECUIV^ |NTEST1NAL Ap< _JDIGESTIVE GLAND 



MADREPORITE 



GUI-LET- 



AMBULACRAL-RING "TSjERVERING \ " RADIAC NEBVE 



MOUTH RADIAL AMBULACRAL VESSEL 



Fig. 278. Common Star- Fish (Uraster rubens]; cut through disc and one arm to show structure. Diagrammatic 



surface. The tube-feet most likely help in breathing, and some 

 of them in the neighbourhood of the mouth appear to be specially 

 concerned with this function. 



Great interest attaches to what is known as the water -vascular 

 system, a set of organs to which the tube-feet belong, and which is 

 quite unlike anything existing in other subdivisions of the animal 

 kingdom. It consists of a tubular ring surrounding the beginning 

 of the food- tube, and sending a radial vessel along each arm, just 

 below the union of the two rows of ambulacral ossicles. The 

 tube-feet are branches of the radial vessels, but they are also 

 connected with little transparent bladders (ampullae) placed within 

 the cavities of the arms. The fluid filling the water -vascular 

 system is largely sea-water, which is able to get in from the 

 exterior by a stone-canal that runs from the madreporite to 

 the central ring, the grooves in the former being perforated by 

 numerous minute holes. The ampullae, by their contraction, 

 force fluid into the tube -feet, thus causing their protrusion. 

 This method of working a set of locomotor organs is quite 

 unlike anything we have so far met with, and it compensates 

 for the comparative feebleness of the muscular system, correlated, 



