Ectodermal Muscles in a Crustacean

Abstract

THE generally accepted view as to the origin of the musculature of the Crustacea is that it is derived from the mesoderm. In Chirocephalus, while the majority of the muscles arise from the mesoderm, there are others that are definitely of ectodermal origin. The dilator muscles of the proctodæum are examples of this type, and probably with these should be classed the dilator muscles of the sophagus. At the most posterior tip of the body, where the ectoderm folds inwards to form the proctodæal tube, the formation of the dilator muscles can be seen most clearly. Certain ectodermal cells in this region elongate and then, while retaining one end in the outer ectoderm, the other passes inwards with the invaginating ectoderm. This passage inwards is probably not an active migration, but is brought about by the proliferation of the surrounding cells, the latter pushing in between the two ends of the muscle cell. In the elongating cell body one or two fibrils appear that very early divide into segments, giving the typical structure of striped muscle. The final muscle is thus an elongated cell, attached at its inner end directly to the cuticle lining the proctodæum and at the other end to the external cuticle.