hurodtiction 5 



and chorclates. No longer do zoologists seriously consider the annelid or 

 arthropod theories oF chordate origin. There is little question that the cephalo- 

 pods show greatest specialization among molluscs, the insects among arthro- 

 pods, and birds and mammals among chordates. The annelid arthropod- 

 mollusc groups show determinate cleawige; i.e., the hiastomeres arrange 

 themselves in a stereotyped pattern, each cell with a fi.xcd prospective role, 

 whereas among echinoderms and chordates cleavage is indeterminate. In the 

 lirst-mentioned group mesoderm formation begins with a particular cell in the 

 blastula, From which two lateral mesodermal bands arise; in the cchinoderm- 

 chordate group mesoderm arises as an outpouching From the archenteron, i.e., 

 From the endoderm. In the annelid-arthropod-mollusc line the coelom is 

 hollowed out From the mesodermal bands, whereas in the echinoderm-chor- 

 dates the coelom pushes out From the archenteron. In the annelid arthropod- 

 mollusc line the blastopore gives rise to the mouth and the anus is opened 

 secondarilv, whereas in the echinoderm-chordate group the anus de\elops 

 From or in the region oF the blastopore and the mouth opens secondarily. 



Species may be considered as reproductively isolated populations which 

 constitute natural units in taxonomy. To the comparative physiologist species 

 are not rigidly fixed populations; rather the individuals within a species show 

 a range oF potentialities with respect to adaptation to environmental stresses. 

 By ascertaining which adaptations are physiological and which genetic it may 

 be possible to picture the e\'olution oF small groups— the origin oF varieties 

 and subspecies. In its larger aspects comparative physiology supports the well- 

 established lines oF organic evolution. Certainly the biFurcation oF the phylo- 

 genetic tree into two main portions and the specialization oF cephalopods, 

 insects, birds, and mammals as terminal groups are two evolutionary conclu- 

 sions which are amply supported by physiological evidence. 



The characters which are most useFul in establishing phylogenetie relations 

 are not as universal as cellular characters, but they are widespread among 

 many animal groups. Good examples will be Found in the fields oF digestion, 

 visual pigments, nitrogen excretion, and in heart, muscle, and nerve physiol- 

 ogy. 



Summary. Comparative physiology is an integrating subject which pro- 

 vides the evidence For many biological generalizations. It is concerned par- 

 ticularly with Functional adaptations to environmental stresses and emphasizes 

 kind oF animal as one experimental variable. In combination with field and 

 genetic studies, comparative physiology has predictive value with respect to 

 distribution and the evolution oF species. There are many unknowns in this 

 field, and each chapter in this book poses numerous unanswered questions. 



REFERENCE 



1. Hyman, L. H., The Invertebrates: Protozoa through Ctcnophora (1940). New York, 

 McGraw-Hill. 726 pp. ^--^Tri-^ 



