I20 CEPHALOCHORDATA CHAP. 
respectively of what have been called “ fin-rays.” They are short 
rods of gelatinous connective tissue, each enclosed in a lymph 
space. Finally, the bars constituting the walls of the pharynx 
between the gill-slits contain slender skeletal rods which run 
obliquely dorso-ventrally, and are of a stiff, gelatinous nature (see 
Fig. 75, p. 122). This skeletal connective tissue consists in all 
cases of a fibrous deposit or matrix produced by the layer of 
epithelium (ectodermal, endodermal, or mesodermal) which adjoins 
the tissue. 
Alimentary Canal.—This has, as its most noteworthy feature, 
the Chordate characteristic that the pharynx gives rise to the 
respiratory organ (see Figs. 71 and 74, A); and in size and pro- 
minence, both in side view and in sections, the modified pharynx 
of Amphioxus is fairly comparable with the branchial sac 
(pharynx) of many Tunicata (see Fig. 23, p. 51), and might be 
called by the same name. 
The small primitive mouth, at the bottom of the cavity 
bounded by the oral hood (stomodaeum), has a membranous 
border, the velum (Fig. 71, v/), the edges of which are prolonged 
into a circle of 10 or 12 (up to 16 in some species) simple oral 
tentacles turned inwards towards the pharynx (compare tentacles 
of Ascidians, p. 45). 
The pharynx, by far the largest part of the alimentary canal, 
and extending nearly half-way along the body, is more important 
as a respiratory than as a nutritive organ. Its walls over nearly 
the whole extent are perforated by a large, and indefinite, 
number (100 or more on each side) of gill-slits which run on 
the whole dorso-ventrally, but in the contracted condition seen 
in preserved specimens have their lower ends directed obliquely 
backwards, so that a vertical transverse section may cut through 
a number of such shts and the intervening branchial bars (Fig. 
74, A, kb). These bars, and therefore the slits between them, 
are of two orders, primary and secondary, the latter being devel- 
oped later in larval life as downgrowths or “ tongue-bars,” one 
from the top of each primary gill-slit, so as to divide it into 
two secondaries. The primary and the secondary (or tongue-) 
bars can be distinguished from one another by their structure in 
the adult animal (Fig. 75, A and B). 
It must be remembered that these branchial bars, or septa 
between the gill-slits, are not merely portions of the wall of the 
