138 How Living Things Come to be What They Are 



were for many years a puzzle to naturalists, as well as a 

 nuisance to mariners. They were supposed to be related to 

 mollusks; they were supposed to fall from the trees; and 

 many other myths about their origin and relationships grew 

 up. Darwin, by studying the development of barnacles 

 from the egg, showed that they are unmistakably crusta- 

 ceans, crab-like animals which adopt a sedentary life 



Tunicate larva 



Amphioxus 



Lamprey 



Fig. 37. Prophetic of Vertebrates 



The sea squirts are hardly suggestive of anything higher than sea anemones or clams. 

 The larva (above), however, has the beginning of a cord and a dorsal nervous sys- 

 tem. The amphioxus or lancelet is worm-like in appearance but it has a flattened 

 fish-like shape and a definite supporting cord. The lampreys do not develop a bony 

 skeleton, but they look like eels and breathe like fishes. 



under special conditions and come to be in the adult stage 

 quite unrecognizable as crabs. There is sometimes found on 

 the abdomen of crabs a parasite consisting of a sac which 

 sucks up the juices of the host and thrives upon them. This 

 sac is almost shapeless and has no distinct structure by which 

 we might recognize its relationships to other types of animals. 



