136 How Living Things Come to be What They Are 



tity of the earlier stages, however, is impressive; and if it 

 has significance, it must point to common ancestry in some 

 remote past. Strangely enough, some of the fresh-water 

 worms whose eggs develop inside of capsules or cocoons, 

 where they never have a chance to swim about and get food 

 for themselves, also pass through the trochophore stage. 



Fig. 36. Comparison of Mollusk and Worm 



A clam is not readily mistaken for a sandworm. In the early stages of development, 

 however, the mollusks and the segmented worms pass through a curious stage called 

 the trochophore, a. Millions of such trochophores swim about in the water, representing 

 many different species of mollusks, and many different species of worms. As the mollusk 

 trochophore continues its development, it acquires distinct structures, r, and the clam 

 larva, b, with the beginning of its two-valved shell, is readily recognizable. The worm 

 trochophore elongates, d, and gradually acquires a longer body, e, which is at last worm- 

 like and segmented, /. 



