108 



Comparative Animal Physiology 



echinoderms, medium reactions to Livtulus, and high reactions to other anne- 

 lids. Molluscs (gastropods and pelecypods) are closer serologically to annelids 

 than to arthropods.^" The principal antigenic protein in crustacean blood is 

 hemocyanin,*^ and the antibodies react with proteins of other arthropods but 

 not with those of molluscs.^" The crab Pseudocarcinus gigas is related, accord- 

 ing to the precipitin test, to four other genera of crabs (Brachyura) but not to 

 Palinuriis, the Astacura, or Anomura. ^^ Relationships among orthopteran 

 insects have been investigated by reciprocal serological tests on equivalent 

 amounts of tissue proteins.^^ The four families are about equally separated, 

 the average interfamily ratio of heterologous to homologous titer being 11.5 per 

 cent, the average intergeneric ratio among grasshoppers (Acrididae) being 



SIREN 



AMPHIUMA 



NECTURUS 



CRYPTOBRANCHUS 



Fig. 27. Relative serologic relationships of four urodele amphibians. 

 From Boyden and Noble.^" 



28 per cent and the ratio between two species of the genus Melanoplus being 

 54.7 per cent. 



Antisera for cuticular tissue of three species of Ascaris parasitic in mammals 

 react indiscriminately among the three antigens but not with the cuticle of 

 an avian Ascaris.'^'^ Acanthocephala appear to be related serologically to the 

 Platyhelminthes but not to the Nemathelminthes, although some precipitin 

 reaction occurs between the Platyhelminthes and Nemathelminthes. ^- 



The preceding examples are sufficient to indicate the importance of applying 

 immunological reactions to the proteins of comparable tissues from different 

 animals as a test of taxonomic relationship. Why should the antigenic proteins 

 of one species or phylum have such structural similarity that they react recipro- 

 cally with their antisera but not with antisera of more distantly related animals? 

 The answer to this question remains one of the unsolved subtleties of protein 

 structure. 



