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 {120}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).