12 



HEMICHORDATA 



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, f). 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 Chorda ta. 

 The gill-sacs of Balanoglossus follow one another closely, 

 the hind wall of one being in contact with the front wall of 

 the next, and constituting a " branchial septum " (h.s). Both 

 „_ septa and tongue -bars are 



supported by chitinous rods, 

 which are special thicken- 

 ings of the membrane at 

 the base of their epithe- 

 lium. 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 

 — nn 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 Baiauo- tum. In this respect again 



glossus, seen from the inside of the pharynx. , • -i -^ i 



b. Branchial skeleton, consisting of a single ^C have a Similarity be- 



forked bar in each branchial septum {h.s), tween BalanOglosSUS and 



and 01 two bars in each tongue -bar ; g.2J, * i ■ 



gill-pore, opening on the dorsal surface of AmphlOXUS, except that in 



the trunk ; g.s, gill-sac ; s, synapticuium the latter the concrescence 



(only one or two shown) ; t, tongue-bar. The 



arrows indicate the communications of the prOCecds One Step farther, 



phU-nx' ™*^ *^' ''"""' '"^ ™'"' *''' aii<i the two rods of the 



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 



