{121}[Illustration: FIG. 74.—_Branchiostoma lanceolatum_. A, transverse section of the pharyngeal region. _a_, Dorsal aorta; _b_, atrium; _c_, notochord; _co_, coelom; _e_, endostyle; _g_, gonad (ovary); _kb_, branchial septa; _kd_, pharynx; _l_, liver; _my_, myotome; _n_, nephridium; _r_, spinal cord; _sn_, _sn_, dorsal and ventral spinal nerves. B, Transverse section of the intestinal region. _atr_, Atrium; _coel_, coelom; _d.ao_, dorsal aorta; _int_, intestine; _myom_, myotome; _nch_, notochord; _neu_, spinal cord; _s.int.v_, sub-intestinal vein. (From Parker and Haswell's _Zoology_. A, From Hertwig, after Lankester and Boveri; B, partly after Rolph.)] It must be remembered that these branchial bars, or septa between the gill-slits, are not merely portions of the wall of the pharynx, but are in a sense portions of the body-wall as well, and correspond in nature, though not in number, to the visceral arches in a Vertebrate lying between the visceral clefts which open on the exterior. In the adult Amphioxus the clefts in the wall of the pharynx do not open directly to the exterior, but into the peribranchial cavity or atrium, which, however, is only formed at a late larval period as an invagination or enclosure of ectoderm. Previous to that the first formed gill-slits opened to the exterior in Amphioxus (see larva, Fig. 86, p. 134), just as they do in a fish or a young tadpole. The atrial cavity is therefore, from its origin, lined by ectoderm, and the outer surface of a branchial bar is virtually a part of the outer surface of the body. It is only natural then to find that each bar contains a small section of the coelom in its interior, communicating dorsally and ventrally with other parts of that cavity (see Figs. 75 and 76). There are also blood-vessels which run in the branchial bars and their junctions. The greater part of the epithelium covering a branchial bar is pharyngeal epithelium or endoderm {122}(Fig. 75, _br.ep_), but the external, wider, non-ciliated cells (Fig. 75, _at.ep_) are ectodermal cells lining the atrium. The gelatinous skeletal rods in the primary bars are forked ventrally, while those in the secondary bars are simple; and there are other points of detail in which the two kinds of bar differ. These bars are obviously more numerous in the adult than the myotomes, but in the young larva the first formed gill-clefts are metamerically arranged, and then later they increase greatly in number. It is the cilia covering the pharyngeal epithelium on the branchial bars, possibly aided by the ciliated tracts of the oral hood, which cause the current of water already alluded to. [Illustration: FIG. 75.—Transverse sections through primary (A) and secondary (B) branchial bars of Amphioxus. _at.ep_, Atrial epithelium; _bl.s_, blood spaces or "vessels"; _br.ep_, branchial epithelium; _coel_, coelomic cavity in primary bar; _sk_, skeletal rods. (From Willey, after Benham.)]