A histological study of Stenostomum virginianum

Abstract

NINETEEN FIGURES INTRODPCTIOSAn understanding of the mesenchyme of the lower metazoans is essential to an interpretation of the complex sustentative tissues of the higher invertebrates and vertebrates. The rhabdocoels provide excellent material f o r such an investigation inasmuch as the animals represent the most primitive Bilateria and are one of the lower groups which have cells between the body wall and intestinal sac. Because of the phylogenetic and physiological importance of the mesenchymal derivatives of the primitive Bilateria, a recognition of their cell-types and disposition is important. The present study, then, consists of the results of an anatomical investigation of the rhabdococl Steaostoinuin virginianuwa Nuttycombe 1931, a member of the Class Turbellaria, which comprises the more primitive Bilateria.The original intention was to restrict the work to the mesenchymal elements ; however, preliminary work on the histology of the animal revealed that a detailed account of the general histology was needed. For this reason, a detailed description of the epidermis, basement membrane, muscular, digestive, and excretory systems, as well as the mesenchyme of S. uirginiamm follows.

Morphology of the Turbellaria at the ultrastructural level

“…These electron microscopical findings parallel the conflicting light histological observations about the basement membrane in the Catenulida (e.g. well developed basement membrane in Stenostomum virginianum, see Pullen 1957, and lack of basement membrane in Rynchoscolex, see Reisinger 1924). In all cases studied so far (with the possible exception of Stenostomum virginianum) the intercellular matrix at the base of the epithelium if present -is extremely weakly developed.…”

The fine structure and function of the adult epidermis of two skin parasitic monogeneans, Entobdella soleae and Acanthocotyle elegans

“…Thus the covering layer of these monogeneans resembles that of the endoparasitic digeneans and cestodes so far described (Braten, 1968;Burton, 1966;Charles & Orr, 1968;Erasmus, 1967;Lumsden, 1966;Morris & Threadgold, 1967;Threadgold, 1965). The fact that the monogenean outer layer is a living epidermis and not a secreted cuticle, as the literature suggests (Bychowsky, 1957) is really to be expected in a group said to be related to the rhabdocoel turbellarians on one hand (which have a cellular epidermis (Pullen, 1957)) and the cestodes on the other (see Llewellyn, 1965). A condition actually approaching that found in the epidermis of Entobdella soleae and Acanthocotyle elegans occurs in the acoel turbellarians des-Fine structure of the monogenean epidermis 47 cribed by Dorey (1965) and Pedersen (1964); here, as in most turbellarians, the epidermis is cellular but the nucleated regions of these cells are situated between and beneath the integumentary muscle layers.…”

A new group of interstitial worms, Lobatocerebridae nov. fam. (Annelida) and its significance for metazoan phylogeny

“…An intercellular matrix is only lacking between these cells in the Acoela, Nemertodermatida and Catenulida, groups which also lack a basement membrane (see above); only some Catenulida contain large, intercellular spaces filled with fluid (e.g., Borkott, 1970, Pullen, 1957, presumably an intercellular body cavity). The intercellular matrix in Macrostomida and Haplopharyngida is finely fibrous to granular and often indistinguishable from the thin basement membrane (see footnote 3).…”

“…Doe). However, with the exception of some Catenulida (see Borkott, 1970) Turbellaria have generally 3 classes of cells filling the body cavity between body wall and gut: (a) insunken gland-cell bodies of epidermal, pharyngeal and genital glands (Tyler, 1976, for Acoela, Nemertodermatida, Haplopharyngida, Macrostomida, Lecithoepitheliata, Tricladida, Polycladida, Rhabdocoela; Sterrer and Rieger, 1974, for marine Catenulida;Doe, 1978, for pharyngeal glands of Catenulida, Haplopharyngida and Macrostomida; own unpublished observations on male reproductive glands in Macrostomida); (b) stem cells, neoblasts or replacement cells (e.g., Moraczewski, 1977, Doe, 1978Doe, 1978, for Haplopharyngida;Petersen, 1964, for neoblast-type cell in Convoluta, Acoela;Petersen, 1966 for neoblast-type cell in Polycladida; Petersen, 1961, Hay andCoward, 1975 for Tricladida;Dorey, 1964, for replacement cells in Acoela; Westblad, 1949 for replacement cells in Meara, Nemertodermatida); (c) one to several types of so-called parenchymal cells (e.g., in Catenulida: see alveolar cells, Reisinger, 1924 andMoraczewski, 1977, Fig. 9; granular strand cells, Sterrer andDoe andindifferent cells, amoebocytes and histiocytes, see Pullen, 1957, Borkott, I970; in Acoela: dense and light ramified parenchymal cells, see Petersen, 1964;vacuolated cells, see Ax, 1966vacuolated cells, see Ax, , D6rjes, 1968pigment cells, see D6rjes, 1966 Hay and Coward, 1975).…”