PHYLUM ECHINODERMATA 



203 



/nferambu/acra/ » 

 p/afes ^ 



Ambu/crcrcf/ 

 p/afes 



of plates running from the oral to the aboral surface. These plates are 

 divided into five pairs of ambulacral rows, between which are an equal 

 number of pairs of interamhulacral rows. The ambulacral rows are 

 perforated for the exit of tube feet and correspond to those in the ambu- 

 lacral groove of the starfish, while the interambuiacral rows would corre- 

 spond to the interradial plates of 

 the starfish (Fig. 110). One may M^^'-^P^^'^^^ 

 conceive of a starfish being trans- 

 formed into a sea urchin by an increase 

 in the vertical diameter of the body 

 and a shortening of the rays until 

 they disappear. With the disappear- 

 ance of the rays the ambulacral 

 bands I'un up around the side of the 

 body and terminate near the aboral 

 pole. The mouth of the starfish is 

 simply an opening in the center of a 

 soft perioral membrane; in the sea 

 urchins, however, it is provided with 

 five converging teeth. These are set 

 in a complicated skeletal box, pentag- 

 onal in shape and known as an 

 Aristotle's lantern (Fig. 111). This is 

 made up of numerous ossicles, lies 

 within the body, and contains muscles 

 which move the teeth. The food of 

 sea urchins consists of algae, which 

 they remove from the surfaces of rocks 

 with their teeth. 



Respiration in sea urchins usually 

 takes place by ten branched pouches 

 arranged in a circle around the mouth. 



mi . 1 /• J 1 • 1 J. i_ Fig. 110. — Dried shell of a sea urchin 



The tube feet are also said to be oi the genus a rbada. shows the arrange- 



respiratory 



exceedingly long if the spines on the 



surface of the animal are long, since of its attachment. Dried pedicellariae 

 ,1 11 J J.I, • n^u <ire still attached to this membrane. 



they reach beyond the spines. Ihe Natural size. 



tube feet are used both in locomotion 



and in holding to surrounding objects. In locomotion the spines are 



used to prevent the pull of the tube feet from rolling the animal over 



and also as levers to help pry the animal onward. The pedicellariae 



of sea urchins are on a stalk and usually have three jaws. 



Sea urchins differ in the length and number of the spines. The cake 

 urchins and sand dollars are exceedingly flat forms with numerous very 



The latter may be ment of plates. .4, the aboral surface. 



B, the oral surface, with the perioral 

 membrane torn loose for about two-thirds 



