356 APPLIED BIOLOGY 



plate and the middle of the arm opposite will divide the body into 

 equal halves with two and a half arms each. 



On the lower surface of the central disc is the small mouth, and 

 radiating from it is a groove on each arm. Along the grooves are 

 the suckers or "feet" by means of which the animal crawls, clings 

 to objects, and obtains food. 



Internally, there is a central stomach and digestive tubes radiating 

 out to the arms. A central nerve-ring has a branch to each arm. 



There is a central ring tube with a branch tube to each arm where 

 smaller branches end in the hollow suckers. This tube with its 

 branches is filled with water, which enters at the small pore-plate on 

 the upper side. This water-vascular system is useful in respiration 

 and in locomotion. 



307. Other Echinoderms. It will be profitable to ex- 

 amine specimens and pictures of other types of echinoderms 

 for the sake of general acquaintance. Only extensive study 

 would elucidate their structure and functions. The forms 

 collected along American sea-coasts are " sea-urchins " 

 (hemispherical and covered with long spines, no arms) ; 

 " brittle stars " (starfish-like, but with long and slender 

 arms) ; " sand -dollars " or " sand-cakes " (flattened discs 

 covered with small spines, related to sea-urchins) ; sea- 

 cucumbers (with tough and leathery skin, and group of 

 tentacles at one end) ; sea-lilies or crinoids (abundant as 

 fossils in rocks, and a few living species occur in deep 

 water). 



308. Economic Relations of Echinoderms. The star- 

 fishes feed on oysters and clams, and are often so numerous 

 as to cause great loss to owners of oyster-beds. Since a star- 

 fish has no jaws and an extremely small mouth, it is evident 

 that only by some unusual method of feeding could it eat 

 an oyster. This is accomplished as follows : The starfish 

 stomach is a large thin-walled sac which can be everted 

 through the small mouth, much as one might turn a glove- 

 finger inside out. A starfish fastens its suckers on an oyster, 

 and then the stomach covers the edges of the oyster's shell, 



