The Cellular Biology of the Epidermis and Muscle Tissue of Some Coelenterates and Planarians

Abstract

The results of investigations into body wall structure of two coelenterates and two flatworms are presented. The coelenterates studied are Isactinia olivacea (Hutton, 1878) (Phylum Cnidaria, Class Anthozoa, Order Actiniaria) and Syncoryne tenella (Farquhar, 1895) (Phylum Cnidaria, Class Hydrozoa, Order Hydroida); the flatworms are Palombiella stephensoni (Palombi, 1938) (Phylum Platyhelminthes, Class Turbellaria, Order Tricladida), and Raphidophallus actuosus (Kozloff, 1965) (Phylum Platyhelminthes, Class Turbellaria, Order Acoela). The body wall of Palombiella stephensoni is composed of an outer epidermis, a basal lamina, a thick filamentous layer, muscle layers, parenchyma, and gastrodermis. The epidermis is cellular and ciliated, and rests on the basal lamina. The thick filamentous layer shows staining reactions characteristic of collagen; it is continuous with filamentous material found in narrow extracellular spaces between parenchymal cells and muscle cells. Muscle cells consist of an elongate contractile part attached by a cytoplasmic bridge to a region of cytoplasm containing the nucleus. The muscle fibres occur beneath the thick filamentous layer as an outer circular layer and an inner longitudinal layer, and occur singly as dorso-ventral fibres. The parenchyma fills the body space between the gastrodermis and the sub-epidermal muscle layers. Cells in the parenchyma include fixed parenchymal cells and neoblast cells. The former type are large cells which possess cytoplasmic processes which ramify throughout the parenchyma in a complex manner, the latter type are small cells considered to be important in regeneration. Intercellular spaces in the parenchyma remain of the order of a few hundred angstroms wide. Proteinaceous, and polysaccharide-containing gland cells are present in the parenchyma. These open to the surface through the epidermis. The body wall of Isactinia olivacea consists of an outer cellular epithelial layer (ectoderm), and an inner cellular epithelial layer (endoderm), which are separated by a cellular and fibrous middle layer (mesogloea). Cells present in the ectoderm include supporting cells, gland cells, cnidoblasts and nerve cells. The supporting cells bear microvilli. The gland cells include proteinaceous, and polysaccharide-containing forms. The endoderm is composed of epitheliomuscular cells, gland cells, cnidoblasts, and nerve cells. The muscle fibres of epitheliomuscular cells constitute the circular muscle layer, and in the mesenteries the longitudinal muscles. The products of endodermal gland cells are similar histochemically to those of ectodermal gland cells. The mesogloea gives staining reactions characteristic of collagen. Mesogloeal fibres have a cross-banded fine structure. Basal laminae separate ectoderm and endoderm from the mesogloea. The body wall of Syncoryne tenella is composed of the same three layers as that of Isactinia olivacea: ectoderm, mesogloea, and endoderm. A cuticle of mucoprotein is present on the ectoderm of the polyp. Cells present in the ectoderm include epitheliomuscular cells, nerve cells, interstitial cells, and cnidoblasts. The muscle fibres of the ectoderm constitute the longitudinal musculature of the polyp. The endoderm is composed of epitheliomuscular cells, gland cells, and nerve cells. The muscle fibres of the endoderm are circularly orientated. Certain gland cells present resemble vertebrate pancreas acinar cells in structure and staining reaction. The mesogloea has a filamentous ultrastructure; it exhibits staining reactions characteristic of collagen. Basal laminae separate ectoderm and endoderm from the mesogloea. The body wall of Raphidophallus actuosus consists of a cellular ciliated epidermis, a cellular peripheral parenchyma, and a cellular digestive parenchyma. Intercellular spaces are of the order of a few hundred angstroms wide, and no intercellular material is demonstrable. The peripheral parenchyma contains muscle cells, fixed parenchymal cells, and gland cells. In this last tissue, as in the digestive (central) parenchyma, the cells are wedged obscurely into one another. The similarities and differences in body wall structure evident between these animals are used as a basis for discussion of the relative evolutionary positions of the phyla Platyhelminthes and Cnidaria, This discussion follows guide-lines indicated by a consideration of current biological concepts important in the classification of lower metazoan animals. It is concluded that the Platyhelminthes exhibit a type of body wall pattern which is a structural advance over that shown by the Cnidaria. Phylogenetic theories which derive the Cnidaria from the Platyhelminthes are regarded as untenable. Two theories are accepted in principle as possibilities. Firstly the theory that the Cnidaria gave rise to the Platyhelminthes; secondly that the Cnidaria and Platyhelminthes arose independently. The latter theory is in my opinion the better theory because it recognizes that structural organization in lower metazoan phyla may be due to independent evolution.