INVERTEBRATE EUCOELOMATES 



259 



those spawning, development of the fertiUzed egg 

 usually produces a free-swimming larva that eventu- 

 ally metamorphoses to the adult form. A few, chiefly 

 polar or deep sea species, keep the young in brood 

 pouches until the adult form is attained. 



This group of echinoderms, like most other echino- 

 derm classes, is of little direct use to man in the 

 United States. Raw or cooked urchin gonads are 

 eaten regularly by people of South America and the 

 Mediterranean. The American Indian once recog- 

 nized the tastiness of the same glands. - 



CLASS ASTEROIDEA (Sea Stars or Starfish) 



Diagnosis: body star-shaped; generally five or 

 multiples of five "arms;" underside of each arm 

 (mouth or oral surface) with a deep, longitudinal 

 groove containing tube feet (Figure 15.4). 



The starfish are usually seen attached to rocks, 

 but they are also found on sandy and muddy bottoms 



of bays, estuaries, and the deep ocean. Most star- 

 fish are carnivorous, preying primarily on shellfish 

 such as clams, scallops, oysters, and mussels. Star- 

 fish are able to exert considerable pressure on a bi- 

 valve shell in attempting to open it. Opening such 

 shells is either accomplished directly by the starfish's 

 arms or aided by a poisonous, digestive secretion. 

 The starfish then turns the front part of its digestive 

 tract inside out so its stomach can envelop and digest 

 the soft parts of the bivalve. When the nutrients have 

 been extracted, the starfish's digestive tract returns 

 to normal. One can see that the part of the digestive 

 tract beyond the stomach is unused. Other starfish 

 tend to eat anything that is at hand, the only criterion 

 seeming to be that the material is organic; the 

 predator starfish are partly scavengers. 



These echinoderms are also typically bisexual. 

 Both free-swimming and brooding larval stages are 

 found in starfish, as well as in the other classes. In 

 some starfish sexuality is similar to that in certain 



Figure 15.4 Pisaster, a starfish (about average adult size); other starfish range from much smaller 

 to many feet across and display great variation in number of "arms." 



