362 



THE POPULAR SCIENCE MONTHLY. 



five grooves, each extending to the tip of one of the five rays (Fig. ?). 

 On each side of these grooves are many actively moving membranous 

 tubes, which are used for crawling, and are called the pedicels or feet. 

 They are closed at the free end, but communicate by a system of tubes 

 within the body of the animal with the madreporic tubercle. It has 

 been surmised that this tubercle acts as a filter to the sea-water which, 

 with some admixture, forms the liquid circulating in the tubes, and 

 Dr. Romanes has proved the surmise to be correct ; 

 for colored fluid, injected under pressure into any 

 part of the system of tubes, found its way to the 

 madreporic tubercle, and oozed through its porous 

 substance. The tube-feet are thrust forth or with- 

 drawn by being distended with liquid or emptied. 

 With the exception of a few at the tip of each arm, 

 every tube-foot bears a sucker (Fig. 4) ; these suck- 

 ers are pressed closely to a flat surface by filling the 

 tube-feet with liquid ; the pressure within the tubes 

 is then lessened, and the greater pressure of the sur- 

 rounding water holds the suckers fast. They are 

 released by increasing the pressure of the liquid 



Fig. 4. -The Terminal within the tube-feet. 

 Portion op a Tube- rr\^ n i n i t 



foot (magnified). lhe common star-nsh usually crawls in a deter- 



minate direction, the feet on the tip of the foremost 

 ray being used as feelers. In a tank, when the star-fish has ascended 

 the side and reaches the surf ace of the water, it often performs peculiar 

 movements which may be called acrobatic. The animal does not wish 

 to leave its native element in fact, can not do so, because its sucking 

 feet can act only under water neither does it wish to descend at once. 



Fio. 5 Natural Movements op a Star-pish on reaching the Surface cp Water. 



It therefore crawls along the side of the tank, now and then throwing 

 back its uppermost ray or rays to feel about for rocks or sea-weed 

 (Fig. 5). If it finds any solid support it will very likely attach its 



