Nielsen, C., Pedersen, K . J . 1979. Cystid structure and protrusion of the polypide in Crisia (Bryozoa, Cyclostoinata) . (Marine Biological Laboratory, Helsingar and Institute of General Zoology, Copenhagen, Denmark.) -Acta 2001. (Stockh.) 6 0 ( 2 ) : 6 5 4 8 .T h e ultrastructure of the cystid of Crisia eburnea has been studied. The cystid wall comprises an outer periostracum, a calcified layer and one inner cell layer, the ectoderm. The membranous sac, which consists of an outer basement membrane, a series of very thin annular muscle cells and an inner layer of epithelial cells, is interpreted as the detached mesoderm of the cystid wall. Accordingly, the atrial sphincter and the generally eight branched, longitudinal muscle cells connecting the terminal membrane and the membranous sac are interpreted as ectoderm. The membranous sac is attached to the cystid wall with two lateral ligaments and at four abfrontal areas: 1) a wide distal area, 2) an area at the origin of the retractor muscles, 3 ) a small area between 1 and 2, and 4) a small basal area at the origin of a pair of small muscle cells attached to the lowermost part of the caecum.We infer that the protrusion of the polypide is caused by a sequential contraction of the annular muscle cells of the membranous sac, starting basally and aided by the contraction of the longitudinal ectodermal muscle cells.

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Head blastemas in regeneratingDugesia tigrina (Planaria) have been studied light microscopically and electron microscopically. Acid phosphatase activity has been followed in early blastemas using a light microscopical cytochemical method. The possibilities of a collagen synthesis inhibiting substance α-α'-dipyridyl in analyzing fibrillogenesis in planarians have been explored.Following a brief discussion of the neoblast concept the general organization and characteristics of the blastema are described. Regeneration of the muscle-connective tissue filament system including the subepidermal membrane is analyzed in detail. It is stressed that the muscle cells, the filamentous sheaths and the subepidermal membrane in planarians should be visualized as a mutually dependent, integrated system. The hypothesis is proposed that neoblasts differentiate into myoblasts which both synthesize myofilaments and collagen. Collagen forms the filaments of the subepidermal membrane-muscle sheath system. No certain interference with collagen synthesis and secretion could be demonstrated in the experiments involving α-α'-dipyridyl.There was no evidence for significant changes in the activity and pattern of acid phosphatase during early stages of regeneration.The problems concerning the existence of neoblasts, their participation in regeneration and their origin (stock cell or result of a dedifferentiation process) are discussed.

Employing transmission electron microscopy, observations were made on epidermis, muscle cells and connective tissue systems, with special emphasis on extracellular matrix components (ECM), in two rather primitive turbellarians: Stenostomum sp. (Catenulida) and Microstomum lineare (Macrostomida). In Stenostomum the only ECM components found are basal laminae, predominantly situated subepidermally. In Microstomum ECM is well developed and connective tissue filaments abundant in conspicuous extracellular spaces. It is uncertain whether basal laminae exist. The finding of basal lamina structures as the only ECM component present in Stenostomum makes it now possible to establish a complete ECM and connective tissue hierarchy in turbellarians, ranging from a purely cellular type with no ECM present to systems dominated by ECM and very similar to loose connective tissue in vertebrates. Comparative aspects of ECM and connective tissue systems in turbellarians are discussed in addition to the difficulties and ambiguities regarding definition and nomenclature of basal matrices as basal laminae and subepidermal membranes.

Fine structural observations on the extracellular matrix (ECM) and connective tissue system of the enigmatic vermiform animal Xenoturbella bocki have demonstrated a complex and interesting organization of the ECM. Most conspicuous is the subepidermal membrane complex (SMC), and the major part of the ECM is present in this structure, which consists of a limiting basal lamina on each side of a central thick filamentous layer, probably of a collagenous nature. Distinct anchoring filaments are found as part of the SMC. A basal lamina is also observed in connection with the gastrodermal cells. The mesoderm is represented by a loose, ill‐defined parenchyma, without filaments. The SMC is discussed in relation to subepidermal basal matrices occurring in turbellarians and enteropneusts, the two groups to which a relationship to Xenoturbella previously has been suggested. Comparisons between the ECM in Xenoturbella and in turbellarians do not support the notion of a relationship between these groups. Conversely, the present study strengthens previous indications of distinct similarities between certain characteristics of the epidermis and the SMC present in both enteropneusts and in Xenoturbella.