18 W. BLAXLAND BENHAM. 



hitherto they have been side by side, or rather anterior and 

 posterior ; but now (fig. 29) one (No. 2) lies outside the other 

 (Nos. 1 and 3), which occupies the normal position. I was 

 unable to track the rods quite to their junction with the floor 

 of the pharynx owing to the imperfection of some of the sec- 

 tions in this series. 



Most of the other abnormal bars contained two rods, but I 

 did not trace them all out from top to bottom, so that I am 

 unable to state whether they always contain, at some part of 

 their length, three rods. But I am inclined to answer this in 

 the negative ; so frequently are there only two rods that I 

 think this is the more " usual" abnormality. There is another 

 triple-rod bar on the opposite side of the pharynx, at about the 

 same level as the one described, and usually, as far as I could 

 observe, the abnormal bars are opposite. 



A bar with a double rod might conceivably be- produced by 

 closure of a primary slit between two primary bars; but there 

 is no evidence of any formation of a slit and subsequent closure 

 and fusion of the bounding bars. One would expect if this 

 had happened that the ccelom and subepidermic vessel would 

 be doubled, and that the pharyngeal end of the bar would 

 exhibit some peculiarity ; this I did not find to be the case. A 

 triple-rod bar might be explained by an extension of this sug- 

 gestion, namely, that two slits had remained imperforate, 

 whilst the rods had been formed nevertheless. 



EXPLANATION OF PLATES 6 and 7, 



Illustrating Dr. W. Blaxland Benham's paper on " The 

 Structure of the Pharyngeal Bars of Amphioxus." 



In most of the figures the blood-vessels are represented black, but in fig. 14 

 I have drawn the actual appearance presented by the sections. 



FIGS. 1 and 2. Transverse sections of a tongue anl a primary bar, so far 

 diagrammatic in that each figure is a combination of several drawings of 



