THE STARFISH AND ITS ALLIES 207 



water plants. A rapid increase in size takes place at this time ; the young 

 after two weeks at the most go to the bottom and begin their life there. 

 It is estimated that if seaweed should be taken out of the water during the 

 months of June and July in the region of Long Island Sound, enough 

 young starfish would be killed to save over six million clams per week for 

 each wagon-load of seaweed removed. 



Other Echinoderms. — Other echinoderms, which are frequently seen, 

 but which have less economic importance, are the sea urchins, sand dollars, 

 sea cucumbers, and sea lilies or crinoids. 



The sea urchin is found all along our northern coast, living in the tide 

 pools along the shore and also in deep water off the coast. 



The body of the sea urchin is almost hemispherical in general form, and 

 is provided with a large number of long spines, which are movable. Loco- 

 motion is performed by means of the tube feet, as in the starfish, but the 

 spines are used to some extent as levers. Food is ground up by means of 

 a set of five strong teeth, placed just within the mouth. 



The sand dollar is a very much flattened form of starfish, modified to 

 withstand the pressure of the deep water in which it is found. Another 

 deep-sea form is the basket star, a much-branched, five-armed form. Others 

 are the brittle stars, so called from their habit of casting off their arms when 

 disturbed. 



The sea cucumber has a leathery skin, only fragments of lime being found 

 scattered through it. The internal structure is much like that of the starfish. 

 It has the curious habit of ejecting all its digestive organs when disturbed. 



The crinoids were once far more common than they are to-day. Their 

 fossil remains form a large part of some of our limestone rocks. 



Classification of Echinoderms 



Class I. Crinoidea. Mostly extinct forms. Deep-sea attached forms. Example, 



crinoid (sea lily). 

 Class II. Asteroidea. Free mo\dng, usually five-armed echinoderms. Example, 



starfish. 

 Class III. Ophiuroidea. Free moving echinoderms with movable arms distinct 



from disk. Example, brittle star. 

 Class IV. Echlnoidea. Echinoderms with no free arms, spines usually movable 



and well developed. Examples, sea urchin and sand dollar. 

 Class V. Holothurioidea. Soft bodied forms, the skeleton consisting of scattered 



limy spicules. Example, sea cucvunber. 



Reference Books 

 for the teacher 



Dodge, General Zoology, pages 87-95. American Book Company. 

 Parker and Haswell. A Text-book of Zoology. The Macmillan Company. 



