WITH NOTES OX THE WEST INDIAN SPECIES. 323 



By their development, size and vascularity, the tongue-bars of the Enteropneusta 

 obviously constitute, collectively, the essential organ of respiration. In Ampkioxus the 

 functional importance of the tongue-bars is greatly diminished ; they are smaller in size 

 and lower in vascularity than the primary bars and their development is secondary. In 

 Amphioxus therefore the conditions are reversed, the primary or septal bars consti- 

 tuting, collectively, the essential organ of respiration. 



In correlation with the further reduction in the number of gill-clefts and the 

 incorporation of the few that remain into the cephalic complex of craniate Vertebrates, 

 the nephric tubules have been released from the topographical relation which they 

 bear to the gill-clefts and to the tongue-bars in Amphioxus, and the tongue-bars 

 themselves have disappeared as such. As I have suggested on a former occasion 

 there is reason to believe that the tongue-bars have not vanished without leaving a 

 trace behind, but that their degradation from the position of essential organ of the 

 gill-cleft has culminated in their transformation into the primordial elements of the 

 thymus of Vertebrate \ The substance of the tongue-bars has been employed in building 

 up the thymus. 



The cycle of phyletic changes undergone by the tongue-bars of the gill-clefts may 

 be epitomised as follows: — 



IX. Pak.vbraxchial Ridges. 



In the larva of Amphioxus tin endostyle is seen to be composed of two halves, 

 right and left 2 . From the anterior ends of the horns of the endostyle a pair of 

 sharply denned ciliated band — the peripharyngeal bands — arch round the anterior wall 

 of the pharynx until thej reach the dorsal side of the latter, whence they proceed 

 backwards to the end of the branchial region. In the adult the dorsal pharyngeal 

 wall becomes modified into the hyperpharyngeal groove and the ciliated bands merge 

 into the walls of this groove. In the adult therefore it is impossible in surface view 

 to see the dorsal recurrent portion of the bands, but it is, at least in young adults, 

 eminently possible to see the anterior arcuate peripharyngeal portion of the ciliated 

 bands 3 . 



The parabranchial ridges (oesophageale Grenzwulste) of the Enteropneusta are 

 likewise ciliated tracts (without the histological differentiation observed in the endo- 

 stvle) which lie at the base of the gill-clefts and arch round in front to unite in 

 the epibranchial band (PI. XXVIII. Fig. 1 A and Text-fig. 7). 



1 A. Willey, Amphioxus and the ancestry of the Vertebrates, 1894, p. 29. Cf. also Pierre de Meuron, 

 rehes ,-»r le dereloppement du Thymus et de la glande Thyroide. Dissertation. Geneva, 1886. De Meuron 

 gives some admirable diagrams to elucidate the origin of the thymus in different Vertebrata. 



- At first upper and lower owing to the peculiar configuration of the larva. 



:; In preserved specimens the contraction of the body quite obscures the ciliated bands in ninety-nine cases 

 out of a hundred. 



