ABSTRACT
Arms of a femal specimen of the cyrtocrinid crinoid Gymnocrinus richeri were analyzed by light and electron microscopy. The following structural characteristics have been stated. Epidermal and foodgroove epithelial cells form circlets of microvilli with or without a central kinocilium which are interconnected by a delicate cuticle. The foodgroove epithelium parries highly graded enrouement of pinnules and arms by displacement of apical cell parts. In II Br 1 to I Br 1 the foodgroove becomes a closed tube with narrow lumen. Sacculi are absent, small sacculus-like structures are observed inside the foodgroove epithelium as well as in the peri-tentacular connective tissue, where also bacteria housing cysts occur. Somatocoelomic structures are highly reduced. Hyponeural nerves are absent. Bullet-shaped inclusions of wandering cells display unusual dotted or striped patterns. Interbrachial muscles are feeble; their fibres vary greatly in size and internal structure; the myofilament arrangement fits neither to a “striated” nor a “smooth” type of muscle cell. Ovaries open via a gonoduct at the pinnular base. Proximal to II Br 1 the rhachis becomes multiplied and is not to be detected in I Br 1.
