IN RELATION TO ZOOLOGICAL DISTINCTION. 65 



forms, which were not crustacean and not arachnid, but gave origin both 

 to the modern-day crustaceans and arachnids. The history of rocks further 

 shows that these ancient fishes, when they first appeared, resembled in a 

 remarkable manner members of the palseostracan group (trilobites, higher 

 scorpion and king-crab forms), so that again paleontologists have found 

 great difficulty in determining whether a fossil is a fish or an arthropod. 

 Fortunately, there is still alive on this earth one member of this remark- 

 able group the Limulus, or king-crab. * * * There are no trilobites still 

 alive, but in Branchipus and Apus we possess the nearest approach to the 

 trilobite organization among living crustaceans." 



In this connection it is of interest to note that hemocyanin has been 

 found in the blood of Scorpiones (Scorpio) , Xiphosura (Limulus) , Decapoda 

 (Homarus, Astacus, Cancer, Carcinus, Nephrops, Eriphia), and Stomato- 

 poda (Squilla and Maia); and that hemoglobin has been found in the 

 bloods of Diptera (Chironomus, Musca domestica), Ostracoda (Cypris), 

 Copepoda (Lernanthropus) , Cladocera (Daphnia), and Phyllopoda (Apus 

 and Branchipus). 



The great importance of hemoglobin in vertebrate life, as is indicated, 

 for instance, in the fact of its universal presence in every living non-degen- 

 erate vertebrate, suggests that if, as Gaskell contends, vertebrates had their 

 origin from pateostraca, it was more likely from one of the group in which 

 hemoglobin and not hemocyanin is the respirator} 7 pigment of the blood. 



These facts, brought together as they are in so fragmentary and unsatis- 

 factory a way, are nevertheless sufficient to be convincing that the results 

 of detailed inquiry, which has been denied us through lack of time, will 

 prove of the utmost importance in zoological differentiation. 



