554 The Animal Kingdom 



species ; in others they remain attached to the pinnules for a time. They 

 hatch into free-swimming larvae which then metamorphose into adults. 



Some species are able to become free-swimming by breaking off 

 from the stalk. Very little is known of the habits of these interesting 

 forms inasmuch as most of them dwell at great depths. They are abund- 

 antly represented as fossils. The ringlike stalk of these fossils is often 

 found in creek beds. They are familiarly called "indian beads." 



There are two other extinct classes of echinoderms, both of which 

 were stalked forms. Usually it is thought that the other classes of the 

 echinoderms arose from these stalked forms. At first this may seem 

 difficult to visualize, but if one were to imagine the calyx of the sea lily 

 freed from its stalk and turned over, he would have a pretty fair star- 

 fish. In turn, the sea urchin can be imagined as a starfish that lost its 

 arms, while the sea cucumber might be considered simply as an elongated 

 sea urchin without spines ! This is, of course, greatly oversimplified, 

 but does show the essential unity of the varied members of this phylum. 

 They demonstrate vividly that evolution goes in all directions, and that 

 the members of a phylum may radiate into many different forms and still 

 retain a basic body plan. Of particular interest is the fact that these 

 forms are all derived from a bilaterally symmetrical type as evidenced by 

 their dipleurula larvae ; most have become secondarily radially sym- 

 metrical while a few have again attained their ancestral bilateral 

 symmetry. 



SOME PHYLA RELATED TO THE ECHINODERMS 



Two phyla which show affinities to the echinoderm-chordate line 

 are the Chaetognatha and the Enteropneusta. The members of both 

 these phyla have indeterminate cleavage, enterocoelous coeloms, and the 

 mouth arising as a new structure. Like the echinoderms, the members 

 of both these phyla are strictly marine. 



The Phylum Chaetognatha. — These small (about one inch long) 

 transparent animals often form an important and abundant part of the 

 mass of floating animals and plants (plankton) of the ocean surface. 

 During the greater part of the day and night they remain well below the 

 surface but at sunrise and sunset they rise when the temperature and 

 light relationships are apparently most favorable. 



Commonly these animals are known as the arrow zvorms due to 

 their general body shape. The body is divided by constrictions into 

 three distinct regions, the head, trunk, and tail. The tail is postanal like 



