BALANOGLOS'SUS, a worm-like marine animal, the chief representative of the most primitive class of chordate animals, Entero pneusta or Adelocephala. This remarkable creature, the type of its class, combines char acters peculiar to itself, with features remind ing us of the nemerteans, annelids, tunicates and the vertebrate omphioxus, while its free swimming larva was originally supposed to be a young echinoderm. From the fact that the central nervous system lies above a notochord. Bateson placed it next to the vertebrates.
One American species, Balanoglossus auran tiacus, is a long, cylindrical, soft, fleshy worm, footless, without bristles, but with a large, soft, whitish tongue-shaped proboscis in front aris ing dorsally within the edge of the collar sur rounding the mouth. The surface of the body is ciliated. At the beginning of the digestive canal is a 'series of sac-like. folds of which the upper or dorsal portion is respiratory and sep arated by a constriction from the lower, which is digestive, and leads ditectly to the intestine behind. This pharyngeal respiratory portion of the digestive canal has on each side, in each segment, a dorsal sac, the two communicating along the median line of the body. The dorsal respiratory sacs each bear in their walls a delicate chitinous gill-support or arch. Be tween the gill-arches, forming numerous lamellae, are a series of slits leading on each side to openings (spiracles) situated dorsally.
The water passes through the mouth into each gill-iac, and out by the spiracles. The nervous system lies above a short sac regarded as a notochord. There is a dorsal blood vessel, which sends branches to the respiratory sacs, and a ventral vessel. The worm lives in sand at low-water mark from Cape Ann to Charles ton, S. C, also in the Mediterranean.
The life-history of this worm is most inter esting. The young, originally described under the name of Tornaria, was supposed to be an echinoderm larva, though it resembles the lar val Gephyrea and Annelida. It is a transparent, surface-swimming, minute, ciliated, slender, somewhat bell-shaped form, with black eye& specks. When transforming to the worm con dition, a pair of gills arise on sac-like out growths of the oesophagus, and afterward three additional pairs, with their external slits, arise, somewhat as in ascidians. The entire Tornaria directly transforms into the worm, the transi tional period being very short. The body lengthens, the collar and proboscis develop, afterward the body lengthens, the end tapering and becoming much coiled. Consult Agassiz, A., 'The History of Balanoglossus and Tor naria) of the American Academy of Arts and Sciences,' Vol IX, Boston 1873) •, 'The Later Stages in the Development of Balanoglossus Kowalevskii, etc.' (Quarterly Journal of the Microscopical Society, London