Gymnosphaera albida
has been found on the sponge
Sycon ciliatum
in the Menai Straits, North Wales, during the months of May to December. It commonly adopts a sedentary mode of life when cultured, settling with its body in contact with the substratum and its axopodia radiating upwards and outwards all round. At times it floats freely. When sessile it can displace itself, but not by rolling. It is a voracious carnivore. The largest seen had a body size of 510 μm x 320 μm. The body of
Gymnosphaerais
divided into three zones: a central medulla, a cortex and a superficial reticulated pseudopodial layer. The medulla is finely vacuolated and contains an axoplast at its centre. The cortical cytoplasm contains many nuclei, Golgi bodies, polysomes, mitochondria, osmiophilic globules, lipoid spherules and vacuoles of various kinds, but no zooxanthellae. The superficial reticulated pseuopodial layer contains osmiophilic globules and occasional mitochondria. Axonemes radiate from the axoplast to the axopodia, along which osmiophilic globules are generally in motion. In between the cortex and the reticulated pseudopodial layer there is a narrow, extracytoplasmic capsular wall (Sassaki’s line), consisting of a microfibrillar coagulum. The wall is a labile structure, perforating locally to allow the passage of food vacuoles or faeces and vanishing completely in certain conditions. It is evaginated to form a sleeve around the base of each axopodium. The cortex is completely penetrated by a system of clefts, the lumen of which opens here and there into the space containing the capsular wall. The clefts are distinct from the endoplasmic reticulum, cisternae of which are commonly found near the surface of the cytoplasmic tracts. Some of the cortical vacuoles contain organic refractive crystals. The crystals have the shape of crossed rodlets, each rodlet having a thermostable component ensheathing a thermolabile component. Their properties are described. The nuclei are enveloped in a thin layer of cytoplasm, connected by narrow bridges to the adjacent cytoplasmic strands. They generally contain several peripherally arranged nucleoli, each bearing a number of nucleolar organizers. Near the centre of the nucleoplasm there is usually a ‘central chromatin body’. The vacuoles of the medulla are of two kinds, one equipped with a fibrous coat. In the vicinity of an axoneme the coat commonly connects with the microtubules and their cross-bridges. The axoplast has a central ‘ hyalosphere ’ exhibiting a fibrogranular matrix. No tripartite organelle is present therein. The axoplast shell consists of the proximal ends of the axonemes, each enveloped by a fibrous sheath, the fibres coursing around adjacent axonemes, binding them together. The shell thickness is a constant fraction (1/2.5) of the axoplast diameter. The axonemes consist of bundles of parallel microtubules arranged in transverse section in a pattern of alternating rows of hexagons, the microtubules being joined together by 12.3 nm long cross-bridges. The crossbridges are absent at the proximal tips of the axonemes and their number diminishes distally from the axoplast to the axopodia; the microtubule arrangement, however, is retained throughout. The capsular wall of
Gymnosphaera
is not essentially different from the tenuous capsular membrane of certain Acantharia and Radiolaria. The reticular organization of the cortex has its counterparts in other groups. The osmiophilic globules are believed to be responsible for the adhesive properties of the axopodia. The refractive crystals have properties like those of the concretions of some Acantharia and the crystals of
Wagnerella
. The definitions of the terms ‘ axoplast’ and ‘ centroplast’ are considered and the neutral term ‘ centriaster ’ is suggested to include either. Finally the classification of
Gymnosphaera
is considered to be unsatisfactory; it has features in common with the Acantharia and with
Hedraiophrys
, but more information on the ultrastructure and life histories of axopodiate species is required before one can improve the classification of the Actinopoda.