. The Cambridge natural history. Zoology. 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, t). The special interest of this mode of development is that it is identical
. The Cambridge natural history. Zoology. 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, t). 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 being in contact with the front wall of the next, and constituting a " branchial septum" (l).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 Balano- tuiu. In this rCSpCCt again glossus, seen from the inside of the pharynx. -, " '1 "f 1 h, Branchial skeleton, consisting of a single ^^'^ ^^^^'^ ^ SUnilarity DC- forked bar in each branchial septum (), twCCn BalanOgloSSUS and and of two bars in each tongne-bar; , i -i • i. +.1, <. • gill-jwre, opening on the dorsal surface of AmplllOXUS, except ttiat m the trunk ; , gill-sac ; s, synapticulum the latter the COncreSCCnCC (only one or two shown); t, tongue-bar. The p i arrows indicate the communications of the proCCedS One Step lartlier, gill-sacs with the exterior and with the ^^^^ ^hc twO rods of the pharynx. tongue-bar unite, like thos
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
