3 o8 



THE ORIGIN OF VERTEBRATES 



surrounds the throat ventrally and laterally, and is the most bulky 

 muscle in the suctorial apparatus. 



The velar or mandibular nerve supplies in Ammoccetes the 

 muscles of the lower lip. In Petromyzon it supplies also the 

 longitudinal muscles of the tongue. The tongue-cartilage first 

 develops in the region of the median ventral tentacle, and there 

 the longitudinal tongue-muscles first begin to develop, not from 

 the rudimentary muscles in the tongue but from those in the 

 lower lip region. 



In Ammoccetes the tentacular nerve supplies the rudimentary 

 muscles in the tentacles and the muscles of the upper lip. The 

 latter disappear entirely at transformation, and in Petromyzon the 

 tentacular nerve supplies the circular, pharyngeal, and annular 

 muscles, which are derived from the rudimentary tentacular 

 muscles. 



For the convenience of my reader I append here a table showing 

 my conception of the manner in which the endognathal and ecto- 

 gnathal segments of the Palaeostracan are represented in Ammoccetes. 

 It shows well the uniform manner in which all the individual 

 segmental factors have been fused together to represent the appear- 

 ance of a single segment (van Wijhe's first segment) in the case of 

 the four endognathal segments, but have retained their individuality 

 in the case of the ectognathal segment. 



