amphioxus (lancelet)
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Habitat, Habit and External Features of Amphioxus

Amphioxus has long been of considerable interest to zoologists and there has been a great deal of investigation into its structure and development. While there is no basic chordate type, Amphioxus, the lancelet is closet to our idea of the primitive ancestors of the craniates. In addition to its chordate characters it has many primitive and many specialized features. It is perhaps best regarded as a modern chordate which has diverged relatively little from a more generalized common chordate ancestor.

The lancelet is found in shallow coastal waters around the Mediterranean, the North sea and the English Channel. It is also extremely common on the Chinese coast near Amoy; at certain seasons of the year, it is so plentiful that it becomes a favored article of food and is sold in the markets. The related genus Asymmetron has a tropical and southern distribution, while Amphioxides  is a pelagic form. The species described here is Amphioxus lanceolatus (Branchiostoma lanceolata). It inhabits localities where the sea-floor is sandy and spends most of its time almost buried, in a vertical position, with the head end protruding. Being a ciliary feeder, it subsists on microscopic creatures in the plankton; the method entails almost ceaseless feeding. Occasionally, it swims, especially at night, by rapid and sinous lateral movements of the whole body. Sometimes it lies passively on its side on the sand. It is able to burrow with great rapidity, proceeding with the pointed end  foremost and the expose part of the body vibrating rapidly from side to side. The sexes are separate and fertilization is external. There is a free- swimming larval stage which shows pronounced asymmetry; this feature persists in the adult, but is not so easily recognized.

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A.lanceolatus has a whitish translucent body, compressed laterally and about 40 to 50 mm in length. It is pointed at both ends. There is a shallow mediam dorsal fin along the whole length; at the posterior end it passes into a slightly deeper triangular caudal fin around the tail. Ventrally, the caudal fin is continuously with a shallow ventral fin which proceeds forward for about a third of the body length to end at a prominent ventral opening, the atripore, the ventral surface is somewhat flattered and at its sides are the two metapleura folds. These pass forward to the anterior end where they merge into the borders of the oral hood. This lies dorsally and laterally over the anterior end its large aperture may be considered to the moth. From its edge, about twenty stiff cirri radiate outwards. The anus lies on the left side near the end of the body just above the ventral fin.

Because of the semi-transparent nature, the major internal structures can be seen. The V-shaped myotomes, with the apex of the V pointing forward, are prominent; there are about sixty. Small fin-ray boxes supporting the dorsal and ventral fins can be seen with a lens. Below these lies the nerve cord, easily discernible by reason of black pigment spots along it. Beneath the nerve cord is the notochord, characterized by its extreme anterior and posterior extension. In immature specimens, the narrow parallel gill bars show that the pharynx extends from a position immediately behind the oral hood almost half-way down the body. But in mature animals, most of the pharynx is obscured by the row of twenty-six gonads on each side. The whole pharynx is covered laterally and ventrally by downgrowths of the body wall enclosing a cavity called the anrium.

The lower surface of the oral hood of amphioxus is wrinkled by a number of finger-like depression with ridges between them. They constitute the ciliated wheel-organ. A deeper pit in the roof of the hood, to the left of the mid-line is called Hatschek’s pit its function is to secrete mucus. At the back of the oral hood is a vertical partition, the velum. This is pierced by a small aperture sometimes known as the mouth. Since, however, it leads directly into the pharynx it is perhaps preferable to refer to it as the enterostome.