The true mouth is a small pore at the bottom of a large vestibule (the stomodaeum), placed at the anterior end of the ventral surface (Figs. 70 and 71), and formed by the "oral hood," which may be a prolongation forwards of the atrial or metapleural folds at each side. The edges of the oral hood bear 12 to 20 pairs of cirri (Fig. 70, cir) or ciliated tentacles (strengthened by skeletal rods), which form a sensory fringe around the opening. The anus (Figs. 70 and 71, an), is asymmetrical, being placed on the left side of the ventral fin, some distance behind the atriopore, and not far from the posterior end of the body. The short region behind the anus and surrounded by the caudal fin may properly be called "tail." The current of water for respiratory and nutritive purposes, and which may carry the ova and spermatozoa to the exterior, usually passes in at the mouth and out at the atriopore, as in the Tunicata. On occasions, however, it is said to be reversed.
General Structure.—The general plan of organisation of the body (see Fig. 71) is that a longitudinal skeletal axis, the notochord (nch), separates a dorsal nervous system (sp.cd) from a ventral reduced coelom (coel), in which lie the alimentary canal (int), the gonads (gon), and other organs. Thus a transverse section of the body (see Fig. 72) shows the typical Chordate arrangement of parts, and is comparable with a transverse section of a tadpole, a young fish, or a larval Ascidian. A peribranchial (atr) or atrial cavity (which is morphologically a part of the external world shut in) lies external to the coelom and body-wall around the pharynx and the greater part of the alimentary canal, and opens to the exterior by the atriopore. As in the Tunicata, the perforations (gill-slits) in the wall of the pharynx (br.cl) open into the atrial cavity and so indirectly to the exterior.
Musculature.—The thick body-wall is largely formed by muscular tissue metamerically segmented into about 60 myotomes (Fig. 71, myom). These muscle-masses, which (as is usual in Vertebrata) are thickest dorsally at the sides of the notochord and spinal chord (Fig. 72, m), are so arranged as to present the appearance in a lateral view of the body of a series of shallow cones (<<) fitting into one another and with their apices directed forwards. The muscle fibres are striated, and run longitudinally along the body from the anterior to the posterior edge of each myotome, so as to be attached at their ends to the two septa of connective tissue which form the boundaries of the myotomes. These septa, the myocommas, are conspicuous features in the external appearance of the body (Fig. 70, B). They are not arranged so as to be opposite one another on the two sides, but the myotomes on the right and left sides alternate, as can be seen in a transverse section (Fig. 74, A, p. [121]).
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 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.