In larval Tunicata and in adults of the group Larvacea the tail is supported by a typical notochord, which is confined to the tail. In all Tunicata except Larvacea all trace of the notochord is lost in the adult. SUBPHYLUM C. CEPHALOCHORDATA. [Illustration FIG. 3. DIAGRAM OF THE SKELETON OF _Amphioxus lanceolatus_ × 3 (after a drawing in the Index collection at the Brit. Mus.). 1. skeleton of dorsal fin. 2. notochord. 3. neural tube. 4. buccal skeleton. 5. branchial skeleton. 6. septa separating the myotomes. 7. skeleton of ventral fin.] This subphylum includes the well-known genus _Amphioxus_[23]. In _Amphioxus_ the skeleton is very simple. It contains no trace of cartilage or bone and remains throughout life in a condition corresponding to a very early stage in Vertebrata. The skeleton of _Amphioxus_ is partly hypoblastic, partly mesoblastic in origin. (_a_) =Hypoblastic skeleton.= The =notochord= (fig. 3, 2) is an elastic rod extending along the whole length of the body past the anterior end of the nerve cord. It lies ventral to the nerve cord, and shows no trace of segmentation. It is chiefly made up of greatly vacuolated cells containing lymph, but near the dorsal and ventral surfaces the cells are less vacuolated. The notochord is immediately surrounded by a structureless cuticular layer, the _chordal sheath_, and outside this comes the mesoblastic _skeletogenous layer_, which also surrounds the nerve cord. The =branchial skeleton=. This consists of a series of chitinous elastic rods which strengthen the gill bars and are alternately forked and unforked ventrally. The forked rods are primary, and are U-shaped in section, the unforked rods are secondary, and are circular in section. All these rods are united at intervals by transverse rods. (_b_) =Mesoblastic skeleton.= The =buccal skeleton=. On each side of the mouth there is a curved bar resembling the notochord in structure. The bars are segmented, and each segment bears a smaller rod which supports a tentacle, the whole forming the buccal skeleton (fig. 3, 4). The notochord is enclosed in a thick =sheath= of connective tissue continuous with a thinner sheath round the nerve cord. The sheaths of the notochord and nerve cord together form the skeletogenous layer, and prolongations of it form the myomeres or septa between the myotomes or segments of the great lateral muscles of the body. The =skeleton of each median fin= consists of small cubical masses of a gelatinous substance arranged in rows (fig. 3, 1 and 7), and serving to strengthen the fins. FOOTNOTES: [19] The name _Balanoglossus_ is used here in its widest sense to include all the Enteropneusta. [20] See W. Bateson, _Quart. J. Micr. Sci._ n. s. vol. XXIV. 1884, p. 208 and later; also E.W. Macbride, _Ibid._ vol. XXXVI. 1894, p. 385. [21] See p. 52. [22] A.T. Masterman, _P.R. Soc. Edinb._ 1895-96, p. 59; and _Anat. Anz._ 1896, p. 266.