STRUCTURE OF THE PHARYNGEAL BARS OP AMPHIOXUS. 105 



therefore^ conclude — as, from a priori reasoning, we should 

 be led to believe — that this septal membrane is double, 

 and is mesoblastic, and not a basement membrane. 



The chitinoid rod of the tongue bar is distinguished, as is 

 well known, from that in the primary bar by the presence of a 

 canal — it is a perforated rod. 



The shape of the section of the rod varies considerably, both 

 in different bars and even in different parts of the same bar, 

 but that represented in this figure may be regarded as the 

 most general shape. Not only the general outline, but the 

 shape and extent of the contained space are included in the 

 above statement as to the variability of the rod. Professor 

 Lankester has already figured several such variations (loc. cit., 

 pi. xxxvi, b), and others will be found on the plate illus- 

 trating this note (PI. 7, figs. 13 — 16). But most gene- 

 rally the rod presents a somewhat triangular section, with 

 rounded angle at the base, and a notch — more or less pro- 

 found — at the apex. This notch forms a part of the internal 

 triangular cavity partially bounded by the septal membrane 

 and occupied by the somatic blood-vessel. 



The chief cavity — the canal — of the rod is similarly more or 

 less triangular in section, rarely round, as Lankester represents 

 it, though this shape does occur. The thickness of the outer wall 

 presents very interesting variations : more usually it is nearly 

 as thick as the sides, but it may be very much thinner (as in 

 fig. 14) ; it may be represented merely by a thin membrane little 

 thicker than the septal membrane, and much thinner than the 

 cutis (basement membrane — Spengel) below the atrial epithe- 

 lium of the primary bar. Further, this rod may be represented 

 by a couple of curved pieces, which do not quite meet externally, 

 so that the curtained cavity is without an outer wall (fig. 15). 

 This is always the case at the points where a synapticulum 

 joins the tongue bar (fig. 31), but it also occasionally occurs 

 elsewhere. 



This cavity frequently appears quite empty, invariably so in 

 my hsematoxylin preparations ; but in sections stained with 

 cochineal, granular matter is very frequently present in greater 



