356 A. T. MASTERMAN. 



tracheal thickenings of Insecta and Vertebrata. The 

 maintenance of an open pharyngeal cleft must be an impor- 

 tant factor in the well-being of this little animal, or the water 

 currents would be diverted into tlie stomach. 



The existence of this chordoid tissue surrounding the 

 pharyngeal cleft is, I think, very instructive in its bearing upon 

 the evolution of the chordate organism. 



Firstly, in regard to the later history of gill-slits in the rest 

 of the Chordata, we note that in every type of vertebrate 

 the gill-slits are supported by branchial bars developed from 

 mesoblast. The mesoblastic cartilaginous skeleton consists of 

 a central axis of cranium and vertebral column, and of an 

 appendicular skeleton of visceral bars. The former are 

 usually regarded as a secondary skeleton, replacing phyle- 

 tically in great part the primary hypoblastic axis or noto- 

 chord ; but so far as my knowledge extends, there has not 

 been described a primary hypoblastic chordoid basis preceding 

 the branchial bars of the gill-slits. This may be partly ac- 

 counted for by the earlier evolution of these bars in com- 

 parison Avith the vertebral column. 



The chordoid skeleton of the pharyngeal clefts of Cephalo- 

 discus can, however, be interpreted as the primary hypoblastic 

 skeleton of this region, which remains the permanent sup- 

 porting structure of the pharyngeal cleft in this type, just as the 

 notochord remains as the permanent supporting axial structure 

 of such forms as Amphioxus and the lowest Vertebrata. 



In other words, Cephalodiscus has the chordoid fore- 

 runner not only of the vertebrate axial, but also of the appen- 

 dicular skeleton. 



In the absence of any ontogenetic evidence, this chordoid 

 condition of the pharyngeal clefts continuous with the true 

 notochordal tissue, renders possible two theories with regard to 

 the evolution of gill-slits (primarily pharyngeal clefts). 



In the first place we may assume that, in earlier forms, cer- 

 tain areas of the pharyngeal wall became chordoid for the pur- 

 pose of support, and that these diverticula came in close 

 contact with the ectoderm, as in the case of the base of the 



