General Characters.—The Cephalochordata (or Acrania, in contradistinction to the Craniata or Vertebrata) are marine, non-colonial Chordata, in which the notochord extends the entire length of the body, running forward into the snout beyond the nervous system. There is no skull, and the notochord is not surrounded by any vertebral column. There are no limbs nor paired fins. There is no exoskeleton, and the ectoderm is a single layer of non-ciliated columnar cells. The mouth is ventral and anterior, the anus is ventral, posterior, and asymmetrically placed on the left side. The pharynx is a large branchial sac, having its sides perforated by many gill-slits, and is surrounded by an ectodermal enclosure, the atrium, which opens to the exterior by a median ventral atriopore. The stomach gives off a simple saccular pouch, the liver, which has connected with it a simple hepatic portal blood system. There is a respiratory circulation, the contractile ventral vessel which represents the heart sending the colourless blood forward to the respiratory pharynx to be purified. The body-wall is segmented into over fifty myotomes. There are numerous separate nephridia which develop from the mesoderm and open into the atrium. The brain remains undeveloped, being scarcely distinct from the spinal cord. There are two pairs of cerebral nerves, and many spinal, in which the dorsal and ventral roots or nerves do not unite. The sense-organs are simple; there are no paired eyes and no auditory organs. The sexes are separate; the gonads are metamerically arranged on the body-wall, and have no ducts: they burst into the atrium. In the development the segmentation is complete, a gastrula is formed by invagination, the nervous system is formed from the dorsal epiblast, the notochord from the hypoblast, and the mesoderm arises from metameric coelomic pouches. The body-cavity is an enterocoele. The gill-slits are at first perforations of the body-wall opening from the pharynx to the exterior, which later become enclosed by the development of the atrium.
Anatomy.
External Characters.—Amphioxus[[112]] is about 1½ to 2½ inches in length, slender, somewhat translucent, and pointed at both ends (Fig. 69). It lives in shallow water and burrows in the sand, head first, with great rapidity. It frequently remains with the anterior end protruding from the sand. When on the surface it lies on one side. It is said to swim freely at night. The head end is rather the thicker, and the anterior two-thirds of the ventral surface are flattened (Fig. 70, A), and may be slightly ridged longitudinally. The lateral edges of this flat area project as metapleural folds (Fig. 70, mt.pl), which begin anteriorly at the edges of the external mouth, and die away in the middle line posteriorly behind a median opening, the atriopore (Fig. 70, atrp). From this point a ventral median fin (vent.f) extends backwards around the pointed posterior end (caudal fin, cd.f), and then forwards along the upper surface (dorsal fin, dors.f) to the anterior end of the body. These fins thus constitute a continuous median fold around a great part of the animal (Fig. 70, B, and Fig. 71).
Fig. 69.—Amphioxus (Branchiostoma lanceolatum) in the Pantano at Messina. (After Willey.)
Fig. 70.—Branchiostoma lanceolatum. A, ventral; B, side view of the entire animal. an, Anus; atrp, atriopore; cd.f, caudal fin; cir, cirri; dors.f, dorsal fin; dors.f.r, dorsal fin-rays; gon, gonads; mtpl, metapleure; myom, myomeres; nch, notochord; or.hd, oral hood; vent.f, ventral fin; vent.f.r, ventral fin-rays. (After Kirkaldy.)
The surface is soft all over, there being no exoskeleton. The epidermis or ectoderm is formed by a single layer of epithelial cells (see Fig. 72, p. [118]), some of which bear sensory processes, while others have a striated cuticular border. There is no general ciliation of the surface in the adult.
Fig. 71.—Diagram of the anatomy of Amphioxus. A, anterior; B, posterior part. an, Anus; atr, atrium; atr′, its posterior prolongation; atrp, atriopore; br, brain; br.cl, branchial clefts; br.f, brown funnel; br.sep.1, primary, br.sep.2, secondary branchial lamella; br.r.1, primary, br.r.2, secondary branchial rod; caud.f, caudal fin; cent.c, central canal; cir, cirri; coel, coelom; dors.f, dorsal fin; dors.f.r, dorsal fin-ray; en.coe, cerebral vesicle; e.sp, eye-spot; gon, gonad; int, intestine; lr, liver; mth, mouth; myom, myotomes; nch, notochord; nph, nephridia; olf.p, olfactory pit; or.f.hd, oral hood; ph, pharynx; sk, skeleton of oral hood and cirri (dotted); sp.cd, spinal cord; vent.f, ventral fin; vent.f.r, ventral fin-ray; vl, velum; vl.t, velar tentacles. (From Parker and Haswell.)