ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 277 



form a kind of basket. The brittle stars, 

 characterized by five long, serpentine arms, 

 can move more rapidly than any other 

 echinoderm. Their tube feet are few in 

 number and are used primarily as touch 

 receptors rather than for locomotion. These 

 animals do not "mind" losing an arm; mild 

 stimulation can cause an arm to be snapped 

 off immediately. The rapid regeneration of 

 a new member makes this a valuable means 

 of escape for these animals. 



The sea urchin (Fig. 12-28) and sand 

 dollar (Fig. 12-29) are also seashore oddi- 

 ties. A common species of the former is 

 usually purple in color and often found ly- 

 ing in the small pockets of rocks, in homes 

 that may be occupied for many years. It has 

 remarkably long spines that are used in 

 locomotion as well as in securing prey. 

 The tube feet which cover the rounded 

 body are also used in locomotion. An 

 interesting organ, characteristic of this 

 animal, is Aristotle's lantern, a compli- 

 cated arrangement of teeth in the mouth 

 that is used for picking food apart. Even 

 small fish are captured and torn to pieces 

 by this effective instrument. In most other 

 details the sea urchin resembles the starfish. 

 The sand dollar is extremely flattened dor- 

 sal-ventrally, so that it resembles a flat disc. 



Otherwise, it possesses the organs common 

 to other forms already described. 



The least known of all the echinoderms is 

 the sea lily, or feather star. It is composed 

 of five greatly branched arms and found 

 only in very deep water, attached to the 

 bottom by means of a stalk. Food is carried 

 down to the mouth by cilia contained in 

 the ambulacral grooves. Its tube feet re- 

 semble tentacles and are probably sensory 

 in function. There are relatively few species 

 in this class today, although ancient rocks 

 show that there were once a great many 

 more, a fact which indicates that they are 

 probably on their way to extinction. 



With the echinoderms and mollusks we 

 have concluded the study of the inverte- 

 brate animals, although some of the primi- 

 tive members of the next group are without 

 vertebrae or backbones and may rightly be 

 considered invertebrates. For the next few 

 chapters we shall be concerned with the 

 last phylum, the chordates, which is the 

 most important of all groups not only be- 

 cause it is a very successful phylum but 

 because it includes man and nearly all of his 

 domestic animals, and that alone is sufficient 

 reason for a careful examination of the 

 group as a whole. 



