STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 15 



their figures, represent the characters of the atrial epithelium at 

 the upper end of the bar correctly, with the exception of Lan- 

 gerhans, who describes each cell here as bearing a flagellum. 



Stieda and Langerhans did not recognise the differences in 

 the length of the cilia round the bar. Schneider is the first to 

 figure the long cilia at the sides, and the remaining authors 

 follow him. Spengel is the first to note this tuft of long 

 cilia at the pharyngeal end ; but both he and Boveri l over- 

 looked the short cilia carried by the lateral groups of cells at 

 this end. 



"With regard to the contained vessels in the primary bar, 

 Langerhans entirely missed the vessel at each end of the 

 septal membrane. Schneider and Lankester saw only one 

 vessel that at the outer end of the bar at the apex of the rod 

 the somatic vessel. Spengel most unaccountably missed 

 this vessel, although he states that he looked carefully for one 

 here, and his figure of the tongue bar (PI. 6, fig. 7) shows a 

 very narrow cleft in its position, whilst he describes the inner 

 or visceral vessel (vn.), which is usually less noticeable than the 

 other. In this respect his figure shows no advance upon that 

 of Stieda, who also saw, but did not describe, the space at the 

 inner end of the bar. 



Boveri, however, observed this vessel (and figures it) in the 

 primary bar (fig. 12, ve. 1), in the position already given to it 

 by Lankester, but overlooked it in the tongue bar (fig. 8). As 

 a matter of fact, as I have stated above, there are three 

 vessels in each bar. Schneider and Lankester found one, the 

 "outer or somatic" vessel; Spengel found one, the "inner or 

 visceral vessel/' but denied the somatic vessel. Boveri found 

 both these in the primary bar, but overlooked the (( outer 

 vessel" in the tongue bar. 



As to the nature of the rod cavity, Stieda and Langerhans 

 leave it aside. 2 Schneider represents (at A, fig. 5) " a blood- 



1 It should be borne in mind, however, that Boveri did not pretend to 

 describe the bars except in so far as they are related to the nephridia. 



2 Stieda regards the granular substance in the centre of the rod (Gg. 2, a) 

 as an axial part of the rod itself. 



