Fig. 73.—Median sagittal section of notochord of an Amphioxus of 32 mm.
In addition to these skeletal layers of connective tissue there is a cartilage-like tract in the oral hood. This is jointed, or made up of separate rod-like pieces, one at the base of each cirrus, into which it sends a prolongation (Fig. 71, sk). The dorsal and ventral fins are supported by single and double rows respectively of what have been called "fin-rays." They are short rods of gelatinous connective tissue, each enclosed in a lymph space. Finally, the bars constituting the walls of the pharynx between the gill-slits contain slender skeletal rods which run obliquely dorso-ventrally, and are of a stiff, gelatinous nature (see Fig. 75, p. [122]). This skeletal connective tissue consists in all cases of a fibrous deposit or matrix produced by the layer of epithelium (ectodermal, endodermal, or mesodermal) which adjoins the tissue.
Alimentary Canal.—This has, as its most noteworthy feature, the Chordate characteristic that the pharynx gives rise to the respiratory organ (see Figs. 71 and 74, A); and in size and prominence, both in side view and in sections, the modified pharynx of Amphioxus is fairly comparable with the branchial sac (pharynx) of many Tunicata (see Fig. 23, p. [51]), and might be called by the same name.
The small primitive mouth, at the bottom of the cavity bounded by the oral hood (stomodaeum), has a membranous border, the velum (Fig. 71, vl), the edges of which are prolonged into a circle of 10 or 12 (up to 16 in some species) simple oral tentacles turned inwards towards the pharynx (compare tentacles of Ascidians, p. 45).
The pharynx, by far the largest part of the alimentary canal, and extending nearly half-way along the body, is more important as a respiratory than as a nutritive organ. Its walls over nearly the whole extent are perforated by a large, and indefinite, number (100 or more on each side) of gill-slits which run on the whole dorso-ventrally, but in the contracted condition seen in preserved specimens have their lower ends directed obliquely backwards, so that a vertical transverse section may cut through a number of such slits and the intervening branchial bars (Fig. 74, A, kb). These bars, and therefore the slits between them, are of two orders, primary and secondary, the latter being developed later in larval life as downgrowths or "tongue-bars," one from the top of each primary gill-slit, so as to divide it into two secondaries. The primary and the secondary (or tongue-) bars can be distinguished from one another by their structure in the adult animal (Fig. 75, A and B).
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 (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.
