STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 11 



with the former by a narrow channel; in this accessory cavity 

 I have sometimes seen a vessel in addition to that in the chief 

 canal, and sometimes I have not detected the latter. I take it 

 that there may be occasional anastomosisbetween the " somatic" 

 vessel in the notch and the skeletal vessel in the canal. 



Comparison of the Tongue Bar and Primary Bar. 



I wish now to compare such a transverse section of a tongue 

 bar with that of a primary bar, so far as regards the Outer 

 end. Compare my figures 1 and 2 and that copied from Boveri 

 (fig. 12) . In the primary bar Boveri describes, as I myself 

 find, three vessels (a) the inner, or visceral, and (b) outer, or 

 somatic, as is the case in the tongue bar, and (c) the third or 

 skeletal (first observed by Spengel) outside the ccelom of the 

 primary bar, between the atrial epiblast and the "cutis" (base- 

 ment membrane of Spengel). This last vessel may project 

 through the cutis into the coelom, and at the base of the bar, 

 where it springs from the endostyle, and where the rod forks, 

 this blood-vessel lies in the angle of the fork, i. e. in the coelom 

 itself (fig. 30, a), as Spengel has figured. In the tongue bar the 

 first two of these vessels are identical with those of the primary, 

 and one can scarcely resist the idea- that the third, or skeletal 

 vessel inside the rod, may correspond with the subepidermal 

 vessel of the primary bar ; it differs from it, however, in one 

 very important point, namely, in the absence of any connection 

 with the subendostylar vessel. But now turn to the rod itself. 

 This is, in the primary bar, made up of two pieces, more or less 

 fused according to the region of the bar (see fig. 30, , b, c], 

 so as to form a triangle, usually with a more or less pro- 

 nounced notch, or linear channel, arising from near the base 

 (see fig. 9) ; or again, as Schneider figured and as I have fre- 

 quently observed, a triradiate split in its centre 1 (fig. 30, c). 

 Outside the rod comes the coelom every one is agreed about 

 that lined by flattened cells; and outside them the cutis, 



1 This usually is due to the softer nature of the central part of this rod, 

 and is not really a cavity : the rod presents irregularly concentric markings, 

 as if shrunk, and is firmer externally. 



