Branchiostoma was long thought to be the source of vertebrates. Today many scientists think Branchiostoma originated as a more mobile animal that assumed its burrowing habit in the sandy bottom of the seashore secondarily.
Branchiostoma is a torpedo-shaped animal with a slightly barbed tail and a tentacled head. Its color results in part from the lack of red hemoglobin in the blood. The animal’s actual color is that of the muscle masses and associated connective tissues as seen through the transparent skin. The fins of Branchiostoma are not especially well defined, but dorsal, ventral, and caudal fins can be distinguished.
A cleared whole mount and representative sections of Branchiostoma reveal the animal’s inner structures without having to resort to dissection. With standard tissue stains, gill slits, notochord, nerve cord, wheel organ, and alimentary tract can be seen.
Notochord and nerve cord. At the anterior end, the notocord extends beyond the dorsally located nerve cord and the tentacled mouth region.
Pigment (eye) spot. The nerve cord terminates anteriorly near a pigment spot, sometimes called an eyespot. Its association with neurosecretory cells suggests that this spot may have some endocrine function.
Myomeres and myosepta. The nerve cord and notochord are flanked laterally by the myomeres, segmented muscle masses which are separated from each other by connective tissue sheets called myosepta. Each myomere is V-shaped when viewed laterally. As a result, it extends along a larger proportion of the side, so that contraction of the myomere produces curvature along an equivalent distance.
Gonads. These are a series of oblong structures along the ventral margin of the myomeres.
Buccal cirri. The mouth of the Branchiostoma is surrounded by buccal cirri, tentacles which prevent large particles from entering the alimentary tract.
Wheel organ. On the inner walls of the oral hood—which encloses the cavity behiud the buccal cirri—is the wheel organ. This ciliated structure funnels the food particles and water stream through the curtain-like velum. Since the buccal cirri are folded over to act as a sieve during feeding, the current of water in the oral hood is prone to stagnation. The cilia of the wheel organ keep this water moving to bring suspended food particles into contact with mucus produced and collected in Hatschek’s pit, a depression in the roof of the oral hood. The food and mucus, swept into a chain-like band by the wheel organ, move through the velum into the pharyngeal gill region.
Branchial basket. The pharyngeal slits in the pharynx or branchial basket do not open directly to the outside. Instead the water swept in with the food passes first into a chamber, or atrium, between the walls of the pharynx and the outer body wall. This water moves posteriorly and escapes the body via an atriopore, which can be seen as an indentation anterior to the musculature of the tail.
Endostyle. At the hose of the diagonal pharyngeal slits is a fold (slightly darker in whole mounts) running the length of the branchial basket. This endostyle produces abundant mucus just as in the urochordates. This mucus, carried dorsally by the ciliated gill bars, joins the mucus band started by the wheel organ at the anterior of the branchial basket. Cells in the endostyle can concentrate iodine into an organic molecule, thyroxine; this suggests homology to cells in the vertebrate thyroid gland.
Intestinal diverticulum. According to Barrington (1965), once the food chain leaves the branchial basket, the currents of cilia in the alimentary tract propel the food and sort out the food particles into the midgut and a midgut structure called the intestinal diverticulum. This structure is sometimes called the hepatic diverticulum or caecum, though it is not related to liver in function. Some scientists have not observed food passage into the diverticulum, although they agree with Barrington on the role this convoluted diverticulum plays in the production of digestive enzymes. The nature of these enzymes led to a theory that the intestinal diverticulum may be the forerunner of the vertebrate’s pancreas.
Ilio-colon ring. At the junction between the mid-gut. or intestinal region, and the posterior alimentary tract, there is sometimes a conglomeration of food particlcs apparent in the cleared whole mount. At this point there is a specialized “ilio-colon ring” of cilia, which moves the food-mucus mass along the gut near the point where digestive enzymes enter the gut. The rest of the alimentary tract is a straight tube, slightly ciliated, which terminates in a highly ciliated anus. It is ciliary action, not muscular contractions, that transports food through the whole gut.
In addition to the production of mucus, the endostyle in Branchiostoma has been shown to have a definite endocrine role as a primitive thyroid gland with the ability to incorporate iodine into several compounds with thyroxine-like activity.
The Amphioxus has a primitive blood vascular system which is closed in the sense of having a complex set of blood vessels. However, it is open in the tissues, so that blood fluid bathes the cells directly in contrast to the capillary system of vertebrates. Motion of the blood is caused by regular contractions of smooth muscle cells along the walls of the larger vessels. Branchiostoma has no respiratory pigment, but this does not present a problem, in as much as the animal’s general respiratory rate on a per weight basis is only about 10 percent of that measured in “cold-blooded” vertebrates.
The presence of excretory cells, the solenocytes, within sacs that empty into the atrium, and the importance of osmotic balance in the estuarine habitat of Branchiostoma, suggest that the blood vessels of the pharyngeal region might be primarily concerned with maintenance of water and salt balance rather than respiration.
Branchiostoma has some unique sensory cells adapting it for survival in a half-buried position on a sandy sea bottom. Around the oral hood are chemoreceptors that prompt an escape response in the presence of stagnant water. Along the nerve cord in the posterior region are ocelli that are sensitive to light. When light shines on the normally buried tail, an escape response occurs. The escape response itself is initiated by a system of giant nerve fibers known as Rhode cells, located near the center of the nerve cord. The axons of these giant nerves make asymmetric connections to the muscle masses, causing jerky, unpredictable movements, very effective as an escape mechanism.
The reproductive system of Branchiostoma is quite simple. There arc male and female Branchiostoma, but under certain temperature and salinity conditions, individuals of each sex can produce both eggs and sperm. Spawning is dictated by temperature and by seasonal factors. Larvae hatch from the fertilized eggs approximately twenty hours after spawning and spend about five months as freely mobile organisms. The larvae then metamorphose into adults.
The notochord of Branchiostoma also deserves comment. It is composed of a tough sheath surrounding vacuolated cells which have considerable turgor pressure: significantly, it also has contractile filament systems running transversely. These contractile systems are located segmentally and are innervated. When a wave of myomere contraction passes down a side of the body as it bends in swimming, a wave of contraction passes down the notochord at the same time. As a result that structure is tensed or made more rigid just at the point of myomere contraction. Apparently this allows the notochord to function more efficiently as a rigid rod that resists shortening of the body during myomere contraction.
What is so surprising about the presence of muscle in the notochord is that the other lower chordates and vertebrates do not have this feature. This means that a truly basic chordate organ, the notochord. has undergone a radical structural and functional modification in a single line of organisms, the Branchiostomids. The reasons are hard indeed to envision, since Branchiostoma is neither a more vigorous nor a more frequent swimmer than an ammocoete or a tunicate tadpole. Furthermore, the muscular notochord also suggests that the cephalochordates are not on the main line of chordate evolution leading toward the vertebrates, unless, of course, the contractile apparatus of cephalochordates was lost secondarily as die true vertebrates appeared.