Fine structure of the apical plate in the larva of the freshwater bryozoan Plumatella fungosa (Pallas) (Bryozoa: Phylactolaemata)

Summary

The fertilized egg of the freshwater bryozoan Plumatella fungosa develops into an elongated ovoid larva, which after embryogenesis in the embryo sack is released into the surrounding water. During the short free-swimming stage the larva is characterized by the presence of several locomotive and attachment cell types. The ectodermal epithelial cells are provided with numerous cilia. The cytoplasm of the epithelial cells contains numerous mitochondria, Golgi bodies, and ribosomes. Gland cells containing vacuoles of flocculent, lightly staining secretion material were observed between the epithelial cells in the anterior pole of the larva. From a neural center beneath the epidermal cells in the anterior pole nerves emerge in a radial direction. These nerves may include up to 30 axons. Beneath the nerves are a lamella of collagenous fibers and a layer of muscle cells. Internally the larva possesses a coelomic cavity bounded by a squamous epithelium. At the anterior pole large, irregular cells rich in rough endoplasmic reticulum and lipid droplets are seen associated with the coelomic wall. It is concluded that the P. fungosa larva is a true planctonic larva, although highly modified and reduced. There is an acceleration of adult features with differentiation of functional zooids within the larval body. This acceleration may be an adaptation to life in freshwater.