12 HEMICHORDATA CHAP. 
view of the animal thus shows a linear series of simple pores, a 
view of the pharynx from the inside appears as in Fig. 5. 
At the hind end of the pharynx the inner opening of the 
developing gill-sac is circular. Slightly further forward the 
dorsal side of the pore is indented into a crescent, which grows 
longer in a dorso-ventral direction, and becomes a U, whose two 
limbs are nearly separated by a mass of tissue, the so-called 
“ tongue-bar” (Fig. 5, 4). The special interest of this mode of 
development is that it is identical with what occurs in Amphioxus 
(p. 120), which is universally admitted to belong to the Chordata. 
The gill-sacs of Balanoglossus follow one another closely, 
the hind wall of one beimg in contact with the front wall of 
the next, and constituting a “branchial septum” (ds). Both 
septa and tongue-bars are 
supported by chitinous rods, 
which are special thicken- 
ings of the membrane at 
the base of their epithe- 
hum. Two rods occur in 
each tongue-bar, separated 
by an interval of body- 
cavity (Figs. 5, 6), and. only 
one rod in each septum. 
Originally of this form 
—nNNn NN— the rods have 
joined in pairs, the united 
limbs forming the single 
== ——— rod of each branchial sep- 
Fig. 5.—Diagram of two gill-sacs of Balano- tum. In this respect again 
glossus, seen from the inside of the pharynx. __ has nilerieals 
6, Branchial skeleton, consisting of a single we have a similarity pe- 
forked bar in each branchial septum (2s), tween Balanoglossus and 
and of two bars in each tongue-bar; g.p, en te A ' . 
gill-pore, opening on the dorsal surface of Amphioxus, except that in 
the trunk; g.s, gill-sac; s, synapticulum the latter the concrescence 
(only one or two shown); ¢, tongue-bar. The 
arrows indicate the communications of the proceeds one step farther, 
gill-sacs with the exterior and with the and the two rods of the 
pharynx. : : 
tongue-bar unite, like those 
of the branchial septum. The latter, the so-called “ primary ” 
skeletal rods of Amphioxus, are forked ventrally as in Balano- 
glossus (Fig. 5). 
In Amphioxus, as in most Enteropneusta, adjacent rods are 
