STEUCTURE OP THE PHARYNGEAL BARS OF AMPHIOXUS. 109 



the cutis (basement membrane of Spengel), or extra-coelomic 

 portion of the rod, as I would regard it. But this apparent 

 forking of the tongue rod is due merely to the passage of the 

 contained blood-vessel out of the rod to the vessels of the 

 adjacent primary bars; so that in reality, as many sections 

 show, the rod of the connecting bar, i. e. the synapticulum, is 

 connected on the one hand with the extra-coelomic portion of 

 the tongue rod, and on the other with the extra-coelomic 

 " cutis " of the primary bar. 



A second difference between the rods is that the skeletal 

 vessel is inside the coelom in the tongue bar, but outside 

 it in the primary bar over the greater part of its extent, 

 though, in the lower part of the latter bar, it is intra-coelomic, 

 as in the tongue bar (fig. 30, a). I have sometimes seen a 

 small cavity outside the rod in the tongue bar, but I have 

 not been able to satisfy myself as to its nature ; it may be 

 merely artifact. 



B. Summary of my Observations and Interpre- 

 tations. 



1. The epithelium of the bar is everywhere only one cell 

 in thickness. 



2. The arrangement of the cells at the pharyngeal end of 

 the bar, both in the primary and in the tongue, is much more 

 definite than previous observers, except Lankester, have 

 figured and described; the central group, contrary to 

 Lankester's opinion, presents two rows of nuclei, and carries 

 a bundle of very long cilia ; the lateral groups carry quite 

 short cilia. ^ 



3. The nuclei at the sides of the bar are oval and not 

 round, and the lowest row has nothing to do with the septal 

 membrane. 



4. There are three blood-vessels in the tongue, as in the 

 * This differentiation of the cilia round the bar may be compared with that 



occurring in the gill filaments of Lameilibranchs, and in the cirri of Bra- 

 chiopods, where there are similarly bundles of long cilia, situated at the sides 

 or angles, and shorter cilia elsewhere. The existence of a skeletal tissue in 

 these cases is a further analogous resemblance. 



