ORAL CIRRI OF SILUROIDS AND ORIGIN OF THE HEAD IN VERTEBRATES. 29 



Gegenbaur (1872) gave a complete account of the structures in 

 Selachii, accepting Cuvier's homology of the premaxillary and maxillary 

 labials, drawing attention, however, to the distinction between the 

 dermal bone and its subjacent tissue. The premaxillary bone must 

 be imagined as developed phylogenetically, not from a cartilaginous 

 substratum, but on such, the bone persisting while the cartilaginous 

 substratum retrogrades and finally disappears. He then set forth his 

 view that the labial arches are to be compared with arches of the inner 

 visceral skeleton or branchial bars. This has been the basis for most 

 of the modern German views. 



Huxley (1876) compared the low r er labials of the frog to the annular 

 cartilage of the lamprey and the "incomplete ring" of Amphioxus, 

 Avhile the upper labials of the frog are the anterior dorsal cartilages of 

 the lamprey. 



Balfour (1882) says of the labials : " The meaning of these cartilages 

 is very obscure ; but from their being in part employed to support the 

 lips and horny teeth of the Cyclostomata and the Tadpole I should be 

 inclined to regard them as remnants of a primitive skeleton supporting 

 the suctorial mouth with which, on the grounds already stated, I 

 believe the ancestors of the present vertebrates to have been provided." 



Howes (1891) has compared the labials of Chimaera and Marsipo- 

 branchs. His figures of Myxinoids are partly erroneous as to facts, 

 being apparently compounded from museum preparations. 



The question of the homology of the labials in Chimaera seems to me 

 to be decided by the work of Vetter, the value of which can hardly be 

 overestimated. He describes in detail the musculature of the labials 

 in Chimaera. 



These structures are worked by four muscles ; the Musculi labiales 

 anterior and posterior, and two portions of the Levator anguli oris. 



The Labialis anterior is supplied by an anterior motor branch 

 of the Trigeminus and corresponds closely to a portion of the Copulo- 

 tentaculo-coronarius muscle of Myxine (Fiirbringer) the " Kopf U' des 

 zweikopfigen Herabziehers des Mundes" (Miiller) which, as I have 

 shown, is innervated by a branch from the premaxillary nerve. 



The Labialis posterior is a portion of the Kopf U of Miiller, and is 

 supplied by a most posterior branch of the motor part of the Trigeminus. 



The portions of the Levator anguli oris are the Retractores 

 tentaculorum of Myxine. 



