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, ). 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 Chordatu. 

 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 " (b.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 

 -"b.s 



OH nn the rods have 

 joined in pairs, the united 

 limbs forming the single 

 rod of each branchial sep- 



FIQ. 5. Diagram of two gill-sacs of Balano- tum. Ill this respect again 



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



b, Branchial skeleton, consisting of a single we have a Similarity be- 



forked bar in each branchial septum (b.s), tweeil BalailOglosSUS and 



and of two bars in each tongue -bar; g.p, . , . , 



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



the trunk ; g.s, gill-sac ; s, synapticulum the latter the Concrescence 



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



arrows indicate the communications of the proceeds One Step larther, 



gill -sacs with the exterior and with the an( j the two ro( } g Q f fa e 

 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 



