Functions of the endostyle in the tunicates

For more than two decades, electrons microscope observations on the endostyle of different species of tunicates have revealed the presence of a densely packed rough endoplasmic reticulum in the glandular cells of the organ. The presence of such a developed ergastoplasm fully supports the hypothesis taht these cells are capable of synthetizing digestive enzymes. Comparative analyses of extracts of the endostyle and the stomach provide evidence that several kinds of enzymes are secreted and mixed with the food cords. The glycasidases are well represented. Other cells of the endostyle, able to bind iodine, are involved in its incorporation into tyrosine end proteins, a biochelmical proof that the endostyle can be considered at the forerunner of the thyroid gland of vertebrates. The endostyle is also an important constituent of the stolon of Pyrosomas and salps (Thalicea), as it givesthese to the whole digestive apparatus of the blastozooids. The unique role played by the endostyle strictly opposes the Thaliacea to the other Tunicata. Another distinctive trait of taxonomic importance is their single protostigma while the tadpoles of ascidians bear at least two. Probably Pyrosomas ans salps branched out very early from the common stock.