422 



THE CHANGING GENERATIONS 



which derives them from a common stock with the echinoderms is best 

 supported by embryological and morphological evidence. The phylum 

 Echinodermata includes the sea lilies, starfishes, sea urchins and their 

 allies. It seems at first sight ridiculous to suppose that there could be any 

 relationship between these sedentary, five-radiate, armored, uniquely 

 constructed animals and the motile, bilaterally symmetrical, metameric 

 chordates. We find, however, that echinoderms possess small, free-swim- 

 ming, bilaterally symmetrical larvae that have curved tubular digestive 

 tracts and external bands of cilia. A very similar larva occurs in the life 

 history of the acorn worm, Balanoglossus, which is a lowly though 



specialized chordate that feeds on 

 the sand and mud of the sea floor. 

 The coelomic cavities in echino- 

 derms appear as five pouches 

 pinched off from the archenteron; 

 in Balanoglossus and the more 

 advanced chordate Amphioxus they 

 develop in the same fashion. This 

 method of coelom formation is not 

 found in other phyla. For these and 

 other reasons it seems quite prob- 

 able that there lived in the pre- 

 Cambrian seas a common ancestor 

 of the echinoderms and the chor- 

 dates, in the form of a small, ciliated, 

 simple-gutted, bilaterally symmet- 

 rical plankton-feeder similar to the 

 larvae shown in Fig. 27.3. If such 

 a common ancestor existed, the 

 chordate phylum is about as old as the rest. Since the ostracoderms all 

 occur in fresh-water deposits, it is presumed that the early evolution of 

 the vertebrates occurred in inland waters during Ordovician and perhaps 

 Cambrian times. 



It used to be supposed that the ancestral vertebrates were boneless 

 types not unlike modern lampreys, and for this reason the heavily armored 

 ostracoderms were dismissed as an aberrant side branch off the main line 

 of vertebrate evolution. Recent anatomical studies have shown how 

 erroneous this interpretation was; the ostracoderms are now firmly 

 established as the ancestors of all later vertebrates, including man, and 

 as such deserve special attention. 



The cephalaspids are the best known of the three orders of ostracoderms, 

 and may serve as examples of the group. By microdissection and the 

 study of ground sections Stensio and other workers have deciphered the 



Fig. 27.3. Resemblance between an echino- 

 derm larva (left) and that of a primitive 

 chordate, the acorn worm (right), suggest- 

 ing a remote common origin of the two 

 phyla. Note the curved tubular digestive 

 tract and the belts of propulsive cilia. 

 {Redrawn from Buchsbaum, Animals with- 

 out Backbones, by permission University of 

 Chicago Press.) 



