194 GENERAL ZOOLOGY 



resting quietly in a current and clam juice is added to the 

 water, it will become restless and move about. If a screen 

 is placed so that the starfish cannot reach the food, it will 

 move as near as possible and try to pass by the obstruction. 



On account of its taste for molluscs, the starfish is one of 

 the worst enemies of the oyster and clam fishermen. The 

 method by which a starfish opens an oyster was long a matter 

 of speculation, . erroneous observation, and dispute. One 

 man thought an acid was secreted which dissolved the 

 shell; another asserted that the starfish took the oyster by 

 surprise and stuck its arm in the gaping shell before it 

 could be closed; other equally fantastic theories had advo- 

 cates. The starfish opens oysters by tiring them out. 

 The tube-feet are attached on either side of the shell and a 

 steady pull is thus brought to bear on opposite sides (Fig. 

 80, A). The oyster has two strong muscles to keep the 

 shell closed, but these finally become tired and the starfish 

 then gains access to the food inside. 



A starfish has a most direct route from food to stomach. 

 After an oyster has been opened, soft folds of the stomach 

 are protruded through the mouth and wrapped about the 

 parts suitable for food. Digestion and absorption take 

 place inside the oyster's shell and outside the starfish's 

 body! There is one disadvantage connected with this 

 method of feeding large amounts of digestive fluids are 

 wasted. A starfish therefore has enormous digestive glands 

 which fill most of the space within the body. The anus in 

 most starfishes is not functional, and, indeed, it has small 

 chance to be, for waste seldom enters the short intestine. 



About the big baggy stomach there is a spacious ccelom 

 and in this cavity the food may pass readily to all parts of 

 the body. A set of tubes (hsemocoel) also carries fluid to 

 certain obscure regions, but the starfish has little need of 

 a separate circulatory system. Respiration takes place 

 through the tube-feet and branchiae. The former are 

 admirably fitted for respiratory purposes; the reservoirs, or 

 ampullae, being in the coelomic cavity, and the external por- 



