THE PROTOVERTEBRATE 25 



omy of living forms, including their larval stages, and 

 the most recent, and in many ways the most plausible 

 theory, is that of N. J. Berrill (1955), who would derive 

 the vertebrates from the free-swimming larval stage of 

 the pre-Cambrian tunicates. 



As our present information stands, however, the gap 

 remains unbridged, and the best place to start the evolu- 

 tion of the vertebrates is in the imagination. The basic 

 features of the forerunner of the vertebrates— which we 

 will here call the protovertebrate'— were long ago laid 

 down in blueprint form by T. C. Chamberlain and re- 

 quire little modification (see Figure 4): it was an elon- 

 gated, spindle-shaped, bilaterally symmetrical form that 

 possessed a stifFened and yet flexible 'backbone' for the 

 support of muscles so arranged as to produce powerful 

 lateral or eel-hke movements of the body to propel it 

 through the water; the backbone and the skeletal mus- 

 cles and the nerves to the latter, were divided into regu- 

 larly repeated segments; the animal possessed a roxmd, 

 jawless mouth by which it fed on plankton, infusoria, 

 and bottom-living plants; it had internal gills supported 

 by rudimentary gill arches, and sense organs and a brain 

 of a sort at the anterior end of the body, while the tail 

 was solid muscle and contained no viscera. We can only 

 guess at its size, but it was probably not more than one 

 or two inches long. Among the invertebrates no such 

 animal is known, for this blueprint resembles neither 

 jellyfish, sea anemone, flatworm, threadworm, earth- 

 worm, snail, oyster, squid, crab, lobster, Limtdus, nor 

 insect, nor any of the larval stages of these forms. The 

 larval stage of the timicates, as proposed by N. J. Ber- 

 rill, presents a reasonable hypothesis, but nevertheless 

 if writers of science fiction want to import life from some 

 extragalactic star, they can do worse than to consider 

 the protovertebrate. 



As we cannot say from what antecedents the verte- 

 brates were evolved, neither can we with certainty name 



