ECHINODERMS. 277 



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 cceca) 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. Retractor muscles serve to drawback 

 the stomach after a meal (see below). 



The nervous system chiefly consists of a nerve-ring 

 around the mouth and a radial nerve in each ray, the 

 whole paralleling the water-vascular system. Eye-spots, 

 one at the end of each ray, are the only specialized sense- 

 organs present. 



The circulatory organs consist of a so-called heart 

 beside the stone-canal, from which vessels run in various 

 directions, the chief portion running between the nervous 

 and water-vascular tracts. The only respiratory organs 

 are the thin- walled branchiae, which are outpushings of 

 the body-cavity upon the dorsal surface. 



The reproductive organs occur at the bases of the arms, 

 one organ on either side of each ray, the ducts emptying 

 in the angle between the arms. From the eggs there 

 hatch out larvae (fig. 90), which are free-swimming and 

 bilateral, and which show not the slightest trace of the 

 radial shape of the parent. 



The starfishes are all marine. They feed largely on 

 clams, oysters, and other molluscs, and are regarded as one 

 of the greatest pests on oyster-beds. The way in which the 

 starfish feeds is interesting. It has no hard parts to break 

 the shell, while the mouth is too small to admit of swallow- 

 ing the oyster. So it folds itself around its prey, attaching 

 its ambulacra to the valves of the shell, and then begins 

 to pull the valves apart. At first this has no effect, but 



