516 Comparative Animal Physiology 



Table 74 shows that phosphocreatine is the only phosphagen in all ver- 

 tebrate muscles and electric organs. Phosphoarginine is the phosphagen of 

 most invertebrate phyla. The distribution in prochordates and echinoderms 

 is particularly useful in tracing the origin of the chordates. Amphioxus con- 

 tains PC exclusively, Balanoglossus contains both PC and PA, and Ascidia 

 contains only PA; this supports other lines of evidence that tunicates are 

 far from the main chordate line. Among the echinoderms the muscles of 

 a holothurian, a crinoid, and starfish have PA only, an ophiuroid has PC 

 only, whereas the jaw muscles of two echinoids contain both PC and PA 



H OH 



1 / 



N - P = O 



/ \ 



/ OH 



HN = C 



\ 

 N - CHo - COOH 



CH3 



Phosphocreatine 



H OH 



I \ 



N - P = O 



/ / 



OH 



HN = C 



'NH 

 (CHO3 

 CHNH2 

 COOH 

 Phosphoarginine phosphate 

 Fig. 234. Structural formulas of phosphocreatine and phosphoarginine. 



in a ratio of about 1 to 2.-«- i^« Larval echinoids have PA only. The enzymes 

 (protein) concerned with phosphorylation and dephosphorylation differ for 

 PC and PA; echinoids contain both enzymes. The presence of both phos- 

 phocreatine and phosphoarginine in echinoderms supports the theory of the 

 relation between this phylum and the chordates. 



Molluscs, arthropods, and coelenterates contain phosphoarginine only. 

 Phosphocreatine has, however, been found in some polychaete worms.-^ 

 There seems to be a tendency for the more active errant polychaetes to pos- 

 sess PC and for the more sluggish tubicolous ones to have PA. A careful 

 examination of many worms-annelids, sipunculoids, flatworms, and others- 

 may give important evidence regarding the point of bifurcation of the phy- 

 letic tree. 



