A tumor in a fresh-water mussel.

INTRODUCTION The description of tumors in invertebrates is rare. Smith in 1934 (4) described a mesenchymal tumor in the oyster (Ostrea virginica.) This tumor had arisen from the pericardium, was nodular or polypoid and consisted of large oval mesenchymal cells with an outer layer of simple columnar ciliated epithelium. A similar tumor was found in an oyster in 1887 by Ryder (2), and another tumor was observed in 1890 by Williams (6) in the fresh water mussel A nodonta cygnea. The tumor from A nodonta cygnea was described as a polypoid adenofibroma from the pallium. Tumors have also been seen in ants by Brun (1), and in Drosophila by Stark (5). Scharrer (3) induced tumors experimentally by cutting the recurrent nerve in an insect. I t is the purpose of this paper to describe a connective tissue tumor with an epithelial covering in the mussel Anodonta implicata. OBSERVATIONS This specimen was found among mussels imported for zoology classes from the Marine Biological Supply House (Woods Hole, Mass.). The shell of the mussel measured 5.8 by 13.1 cm. and the visceral mass 3.3 by 6.0 cm. The tumor arose from the inner labial palp of the right side and measured 0.8 cm. in diameter and 2.0 cm. in length. Its surface was nodular or polypoid, the nodules being of unequal size (Fig. 1). Region adjacent to the tumor.--The inner surface of the palp was covered by folded simple columnar epithelium bearing long cilia. The outer surface of the palp was smooth with no folds and its epithelium was for the most part stratified squamous, but in certain regions columnar. The columnar cells of the inner surface, which were continuous with the tumor epithelium, were long and narrow, with spindle-shaped nuclei. No granules were seen in the cytoplasm of these cells. Scattered among the epithelial dements were goblet cells with large vacuoles. The connective tissue included irregularly arranged fibers of variable thickness. The connective tissue cell nuclei were well stained but the cytoplasm was scant. Among the fibers were numerous masses of connective tissue cells filled with yellow