{118}[Illustration: FIG. 72.—_Branchiostoma lanceolatum_. Diagrammatic transverse section of the pharyngeal region, passing on the right through a primary, on the left through a secondary branchial lamella. _ao_, Dorsal aorta; _c_, dermis; _ec_, endostylar portion of coelom; _f_, fascia, or investing layer of myotome; _fh_, compartment containing fin-ray; _g_, gonad; _gl_, glomerulus; _k_, branchial artery; _kd_, pharynx; _ld_, combined atrial and coelomic wall (ligamentum denticulatum); _m_, myotome; _mt_, transverse muscle; _n_, nephridium; _n.ch_, notochord; _of_, metapleural lymph space; _p_, atrium; _sc_, coelom; _si_, ventral aorta; _sk_, sheath of notochord and spinal cord (_sp.cd_); _uf_, spaces in ventral wall. (From Korschelt and Heider, after Boveri and Hatschek.)] It is by means of these lateral muscle-bundles that the rapid vibration or alternate bending of the body from side to side in swimming or burrowing can be performed. There are usually, on each side, 35 myotomes in front of the atriopore, 14 between the atriopore and the anus, and 11 postanal, making 60 in all: some species have only about 50 myotomes, and some as many as 85. (See {119}Classification, p. 137, where a list of the species with the number of myotomes in each is given.) There are also transverse muscles (Fig. 72, _mt_) extending across the ventral surface in the region of the body enclosed by the metapleural folds, and serving to compress the atrial cavity, and so aid in the expulsion of its contents. Outside the muscular layer of the body-wall the thin integument is formed of a dermal layer of soft connective tissue, covered by the epidermis, a single layer of columnar cells, many of which, especially on the oral cirri, have sensory bristles. SKELETON.—The endoskeleton consists of the notochord and some tracts of modified connective tissue which support various parts of the body. The notochord of this animal is noteworthy amongst Chordata for extending practically the entire length of the body, including the head, from snout to tip of tail (Fig. 71). It lies in the median plane, but nearer the dorsal than the ventral surface (Fig. 72), and has the myotomes at its sides, the nervous system above and the alimentary canal below. It is elliptical in section, and tapers to the two ends. The nuclei of the original notochordal cells are displaced to the dorsal and ventral edges, and the greater parts of the cells, in the adult, are occupied by large vacuoles filled with a fluid secretion, so as to form by their distended condition a stiff elastic structure. This state of the cells, and the appearance it gives rise to (Fig. 73), seen best in young specimens, is very characteristic of notochordal tissue. Around the notochord lies a sheath of connective tissue which is continuous with the similar sheath around the nervous system and with the septa between the myotomes. [Illustration: FIG. 73.—Median sagittal section of notochord of an Amphioxus of 32 mm.]