BRANCH ECHINODERMATA : STARFISHES, ETC. 1 21 



which a central disc and usually five arms are more or 

 less readily distinguishable, the arms being hollow and 

 each containing a prolongation of the body-cavity and 

 contained organs"; the OgJiLurojdea, or brittle-stars, 

 "star-shaped free Echinoderms, with a central disc and 

 five arms, which are more sharply marked off from the 

 disc than in the Asteroidea and which contain no spacious 

 prolongations of the body-cavity ' ' ; the Echinoidea, or 

 sea-urchins, ' ' free Echinoderms with globular, heart- 

 shaped, or disc-shaped body enclosed in a shell or corona 

 of close-fitting, firmly united calcareous plates"; the 

 Holuthuroidea, or sea-cucumbers, "free Kchinoderms 

 with elongated cylindrical or five-sided body, . . . with a 

 circlet of large oral tentacles "; and the Crinoic]ea, or 

 feather-stars, * ' temporarily or permanently stalked Echi- 

 noderms with star-shaped body, consisting of a central disc, 

 and a series of five bifurcate or more completely branched 

 arms, bordered with pinnules." 



Starfishes (Asteroidea). The starfishes feed on other 

 marine animals, especially shell-fish and crabs. They 

 are also reputed to destroy young fish. By means of their 

 sucking-tubes, or tube-feet with sucker tips, they can 

 seize and hold their prey firmly. They do much injury 

 to oyster-beds by attacking and devouring the oysters. 

 When attacking prey too large to be taken into the mouth 

 the starfish everts its stomach over the prey and devours 

 it. The stomach is afterward drawn back into the body- 

 cavity by special muscles. 



Starfishes vary much in size, color and general appear- 

 ance, although all are readily recognizable as starfishes 

 (fig. 22). The number of arms or rays varies from five 

 to thirty or more in different species; some have the 

 interradial spaces filled out nearly to the tips of the rays, 

 making the animal simply a pentagonal disc. In size 

 starfishes vary from a fraction of an inch in diameter to 



