ENDOSKELETON OF BALANOGLOSSUS

A definite endoskeletal system is absent in Balanoglossus but the basement membrane becomes extraordinary thickened and lamellated to perform the function of skeleton. Following are the skeletal elements. 1. Proboscis skeleton It is a’ Y’ shaped structure, partly secreted by the epithelium and partly by coelomic tissue. It resembles to a hyoid apparatus in general appearance. It consists of a median plate in the proboscis stalk continued behind into two narrow horns which extend into the roof of buccal cavity The median plate produced ventral into keel. The median plate lies below the buccal diverticulum. 2. Buccal diverticulum It is short, hollow and stiff tube-like projection extends forward through the proboscis stalk into the proboscis coelom. Bateson (1885) referred to it as alleged notochord. It is termed as stomochord by Willy (1899) and Dawydoff 1948). Hyman preferred to call it as the buccal diverticulum. Its wall is composed of a single layer of tall, slender, vacuolated endodermal cells. Silen (1950) has concluded it as an extension of the preoral-region of the digestive tract. 3. Branchial Skeleton: The walls of the U-shaped gill-clefts are supported by skeletal rods called primary and secondary gill-rods formed by thickening of the basement membrane. 4. Pygochord: Pygochord is longitudinal rod-like structure extending from the ventral side of the intestine to the body wall, its function is not known but it may support the soft abdominal region.

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