276 



THE RISE OF ANIMAL LIFE 



Fig. 12-28. The sea urchins are covered with long spines 

 which aid the long, slender tube feet in moving slowly 

 over the ocean floor. By means of a set of five sharp 

 teeth they are able to tear vegetative and decaying 

 matter to tiny bits which they consume. The aboral 

 and oral views of a young sea urchin (Arbacia) are 

 shown here. 



light, it will contract at once. If given a 

 choice, it will seek out moderate illumina- 

 tion. 



This animal possesses remarkable powers 

 of autotomy and regeneration of lost parts 

 and, in fact, even employs this behavior as 

 a mechanism of defense. If disturbed by an 

 intruder who persists any length of time, 

 the sea cucumber suddenly contracts its 

 muscular walls until considerable pressure 

 is built up within. Then it splits open, al- 

 most explosively, near the anus, everting 

 the respiratory trees which secrete a mu- 

 cous fluid that becomes stringy and tough 

 when it contacts sea water. The unfortunate 

 enemy, usually a lobster, thus becomes 

 hopelessly enmeshed in this mass of threads 

 so that it is no longer concerned with the 



sea cucumber as a prospective meal. The 

 sea cucumber is able to break the trees 

 loose at their base and regenerate a com- 

 plete set within a short time. 



Another interesting group of echinoderms 

 includes the brittle stars and the basket 

 stars (Fig. 12-27). Both forms possess small 

 discs and long, slender, motile arms, the 

 arms of the basket star being branched to 



Fig. 12-29. Sand dollars are close relatives of the sea 

 urchins but are much flattened and the spines are 

 much smaller. Both the tube feet ^nd spines make 

 movement possible in the sand where they live. They 

 feed on the organic matter that is present in the sand. 



The typical five-arm arrangement of the openings 

 on the aboral side through which the tube feet pass 

 is shown in the top picture. These dorsal tube feet 

 are long and are modified for breathing. 



The oral surface in the bottom picture shows the 

 mouth opening in the middle and the anal opening 

 near the edge. 



