. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 178 O. R. ANDERSON ET 2 FIGURE 1. A living Physematium muelleri with prominent intracapsular lobes and a centrally located nucleus. Bar = 1 mm. FIGURE 2. Siliceous spicules on the surface of the central capsular wall of P. muelleri. Bar = 20 nm. are also observed in the regions of the cytoplasmic lobes where the ensembles of encircling mitochondria and dense granules are abundant. The capsular wall possesses numerous fusules (strands of cytoplasm) directed out- ward, forming continuity between the intracapsular lobes an
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 178 O. R. ANDERSON ET 2 FIGURE 1. A living Physematium muelleri with prominent intracapsular lobes and a centrally located nucleus. Bar = 1 mm. FIGURE 2. Siliceous spicules on the surface of the central capsular wall of P. muelleri. Bar = 20 nm. are also observed in the regions of the cytoplasmic lobes where the ensembles of encircling mitochondria and dense granules are abundant. The capsular wall possesses numerous fusules (strands of cytoplasm) directed out- ward, forming continuity between the intracapsular lobes and the extracapsular rhi- zopodial assembly (Fig. 5). The fusule structure resembles that in other large species of solitary radiolaria (, Anderson, 1976, 1983, p. 114), consisting of a thin strand of cytoplasm emerging from a tip of an intracapsular lobe, penetrating the capsular wall to which it is attached, and emerging on the extracapsular side as an electron- dense segment surrounded by a collar-like rim. The rim is perforated by micropores, giving a sieve-like quality to the wall of the rim. Distal portions of the extracapsulum exhibit rhizopodia and digestive vacuoles of varying diameter. Scanning electron mi- croscopic views of the surface of the central capsule (Figs. 10, 11) show that the fusules occur in small clusters distributed over the surface of the central capsule. This is consistent with the transmission electron microscopic evidence showing long spaces of capsular membrane separating groups of fusules. As a contribution to the comparative fine structure of Physematium and its relatives, we have examined the capsular organization of Actissa sp. collected at the same location near Barbados (Fig. 12). The peripheral capsular cytoplasm is more dense than that of Physe, !um, and possesses large peripheral vacuoles near the capsular wall as described earlier by light microscopic investigations (, Haeckel, 1887). These fine structural data confirm one of the
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