The phyllobranchiate gills of the green shore crab Carcinus maenas have been examined histologically and ultrastructurally. Each gill lamella is bounded by a chitinous cuticle. The apical surface of the branchial epithelium contacts this cuticle, and a basal lamina segregates the epithelium from an intralamellar hemocoel. In animals acclimated to normal sea water, five epithelial cell types can be identified in the lamellae of the posterior gills: chief cells, striated cells, pillar cells, nephrocytes, and glycocytes. Chief cells are the predominant cells in the branchial epithelium. They are squamous or low cuboidal and likely play a role in respiration. Striated cells, which are probably involved in ionoregulation, are also squamous or low cuboidal. Basal folds of the striated ceils contain mitochondria and interdigitate with the bodies and processes of adjacent ceils, Pillar cells span the hemocoel to link the proximal and distal sides of a lamella. Nephrocytes are large, spherical cells with voluminous vacuoles. They are rimmed by foot processes or pedicels and frequently associate with the pillar cells. Glycocytes are pleomorphic cells packed with glycogen granules and multigranular rosettes. The glycocytes often mingle with the nephrocytes. Inclusion of the nephrocytes and glycocytes as members of the branchial epithelium is justified by their participation in intercellular junctions and their position internal to the epithelial basal lamina.

The larval tunic of Corella inflata is composed of two cuticular layers, extracellular filaments and ground substance. It lies outside the epidermis and most of it is known to be produced by the epidermis. The dorsal, ventral and caudal fins are specialized parts of the tunic that are essential for larval locomotion. The following hypothesis was tested: Morphogenesis of the larval fins is dependent upon the presence of extraembryonic structures (test cells, chorion or follicle cells) before completion of the late tail bud stage of development. We tested this by dechorionating embryos of Corella inflata and Ascidia paratropa. The operation removes all extraembryonic structures. It was performed mainly on neurula, early tail-bud and late tail-bud stages. Fin formation is inhibited when neurulae are dechorionated but not when late tail-bud or older embryonic stages are dechorionated. Dechorionated neurulae produce all of the major components of the tunic (cuticular layers, filaments and ground substance) but they are unable to form functional fins. At the time of dechorionation, in all experiments, the embryos had no fins. Removal of the follicle cells does not inhibit fin formation. The test cells are known to secrete granular "ornaments" that attach to the surface of the tunic. The fibrous, acellular chorion may serve to contain the test cells and their products or products of the embryo that are not firmly attached. The test cells may induce or control the morphogenesis of the larval fins in ascidians before the late tail-bud stage of development. We suggest ways of testing this hypothesis and an alternative hypothesis.

Abstract. The anterior lateral arteries are paired vessels leaving the anterior end of the lobster (Homarus americanus) heart and proceeding to the antennae and eyestalks, the stomach and hepatopancreas, the gonads, and the thoracic and branchial muscles. These vessels have a trilaminar organization, consisting of a tunica interna with elastic fibrils, a tunica intermedia represented by a bilayered cell mass, and a tunica externa with collagen fibrils. In the tunica intermedia, cells flanking the tunica interna (light cells) show less affinity for basic dyes and electron stains than those flanking the tunica externa (dark cells). Each light cell exhibits an irregularly shaped stress fiber (a bundle of closely packed microfilaments) in the region adjoining the tunica interna. Collectively, these bundles have a circumferential or slightly oblique orientation relative to the lumen of the vessel. The role of the stress fibers is unresolved. If they are static structures, they might contribute to the non‐linear elasticity shown by lobster arteries. If they generate force, and small bundles of microfilaments do diverge from the stress fibers to enter filamentous mats applied to the plasmalemmata, a coordinated contraction of the cells might reduce the luminal diameter and, thus, retard the flow of hemolymph. Coordination of contraction would have to occur in the absence of nerves and without the benefit of communicating (gap) junctions between the light and dark cells.

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