ORAL CIRRI OF SILUROIDS AND ORIGIN OF THE HEAD IN VERTEBRATES. 39 



up the accompanying scheme of the occurrence of these tissues. A 

 stroke between two letters indicates that the tissue is intermediate 

 between two variaties. 





Na. 



Pmx. t. & p. 



Mx. t. 



& Prepal. 



Cor. 



t. & p. Mental t. & p. Subm 



Clarias 



B 





B 



C 





(degenerate) 



Auchenaspis 







B 



C 





A B A B 



Trichomyc- 



C/D 





C/D 



C 



C/D 



A 



terus (young) 













,___^ 



Callichthys 





D 



C/D 



c 



C/D 



D C/D 



(young) 















Chaetostomus 





A/D 



A/B 



c 





A (hard) 



(young) 















Acipenser 





D 



D 



c 







(young) 















Misgurnus 





(rudimentary) 





c 





D Extramental D 



(young) 















Motella 



D 







c 





(replaced Extramental D 



(young) 













by bone) 



Other forms 





C 





c 





C/D C 







(Teleostei 







(Polypterus) (Dipnoi) 



From such a scheme it may be learnt that all parts are independently 

 variable. Root piece and tentacle in Am.phioxus are essentially similar 

 in structure. In Myxinoids the root piece is differentiated from the 

 tentacle by the great development of the intercellular matrix, the 

 hardening of the same, and the degeneration of nuclei and protoplasm. 

 In Siluroids much more complicated differentiations have arisen. The 

 tentacles are not of similar histological nature in different families. 

 The root piece differs from the tentacle in the same individual, the 

 root pieces differ among themselves, some being of precartilage, some 

 of the Myxinoid tissue, and some of true hyaline cartilage. Usually 

 in one animal all the tentacles are of similar histological nature, but 

 nevertheless this is not always the case, e.g., in Motella iricirrata. 



To a certain extent the grade of histological differentiation is a 

 measure of the constancy of the piece. For example, Meckel's cartilage 

 and the hyoid cartilages occur in all the Craniata. The prepalatine 

 piece, the hyaline premaxillary piece of Teleostei, the mental piece of 

 Holocephali and of the Dipnoi are less omnipresent, but still run 

 through whole orders, while less differentiated tentacles are present or 

 absent in different genera. Of course there is no universal rule. 



In view of the extraordinary amount of variation in histology 

 and structure, it is very remarkable that, when tentacles do occur 

 sporadically, they can be referred to certain of definite 6 or 7 pairs. 

 Following the conception of Wiesmann, it would seem that the archi- 



