102 INTRODUCTION TO EVOLUTION 



resemble the larvae of members of Phylum Echinodermata. This phylum 

 includes starfishes, brittle stars, sea urchins, and their spiny allies. There 

 is little about the adults to suggest relationship to, and possible ancestry 

 of, vertebrates. Yet because of the striking larval similarities, especially in 

 the mode of development of the coelom (body cavity), the most widely 

 accepted theory of vertebrate origin is that some echinoderms, or a group 

 ancestral to echinoderms, gave rise to the group of protochordates of which 

 Balanoglossus is a survivor, and that this group gave rise to vertebrates. 

 Amphioxus is more like vertebrates than are the other protochordates; it 

 may be a survivor of a group in the ancestral line or allied to it. 



Within the last few years unexpected corroboration of this theory has 

 come from the field of biochemistry. A complicated series of chemical reac- 

 tions is involved when muscles contract. Involved in one step of the proc- 

 ess are chemical compounds known as phosphagens (recall the impor- 

 tance of phosphorus in transport of free energy in muscle cells, p. 90). 

 In the muscles of vertebrates the phosphagen present has in its composi- 

 tion the substance called creatine, the compound being known as creatine 

 phosphoric acid, or phosphocreatine, or, for brevity, PC. On the other 

 hand, the muscles of most invertebrates have a phosphagen containing the 

 substance called arginine, the resulting compound being known as arginine 

 phosphoric acid, or phosphoarginine, or PA. 



Here we have a clear-cut chemical distinction: vertebrates all have PC; 

 most invertebrates have PA. How do the protochordates fit into the pic- 

 ture? Amphioxus resembles vertebrates in having PC only. Some species 

 of tunicates appear to have PA only, others PC only (Morrison, Griffiths, 

 and Ennor, 1956). Balanoglossus possesses both PA and PC. Thus these 

 protochordates, taken collectively, seem to link vertebrates with inverte- 

 brates. Turning to the latter we ask: Is PC possessed by any groups of in- 

 vertebrates? Some echinoderms possess PC, and some segmented worms 

 (Phylum Annelida) have a substance which is similar and may be the 

 same. Some echinoderms possess PA only, at least one possesses PC only, 

 others possess both PA and PC, as does Balanoglossus. These facts 

 strengthen the idea that the group represented by Balanoglossus is the con- 

 necting link between invertebrates and vertebrates. There is also the pos- 

 sibility that, as suggested by Prosser (1960), PC has arisen more than once, 

 by parallel evolution in independent evolutionary lines (see below). Yet 

 the distribution of PA and PC, particularly when taken together with 

 similarity of larvae, suggests that echinoderms are at least closely related 

 to the ancestors from which the chordates arose. It is entirely possible, 

 of course, that both echinoderms and chordates arose from a common 



