ASTEROIDA (STARFISHES). 



In the starfishes the flattened body is either pentagonal, 

 or has a number of arms, or rays (usually five), giving it 

 the shape of a star. In the body- wall are numerous calca- 

 reous plates, movable on one another. In the axis of each 

 ray, on the side of the body with the mouth (oral surface), 

 are regularly arranged ambulacra! plates, margined on 

 either side by interambulacral plates similarly arranged. 

 In the rest of the surface (aboral surface) no such regularity 

 of plates occurs. The mouth is in the centre of the disc 

 which unites the rays, and is always without jaws or other 

 hard parts. The mouth opens directly into a capacious 

 stomach, the extent of which is increased by gastric pouches. 

 The stomach is also partially divided by a constriction into 

 two chambers, an oral, cardiac, and an aboral, pyloric, 

 division. From the latter a short intestine runs to the 

 aboral pole, where it may open by a vent, but in some no 

 vent occurs. Into the pyloric chamber empty the ducts of 

 five pairs of glands (hepatic caeca) which secrete the digestive 

 fluids, while from the intestine arise from one to five 

 saccular outgrowths, the branchial trees, the function of 

 which is uncertain. 



The organs of locomotion consist of tube-feet or ambula- 

 cra on the oral surface of each arm. These are connected 

 with sacs or ampullae inside the ray, and each of these 

 systems is in turn connected by lateral canals with a 



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