The Fine Structure of the Gregarine Pyxinoides balani Parasitic in the Barnacle Balanus tintinnabulum*†

SYNOPSIS. The fine structure of the eugregarine Pyxinoides balani was studied and compared with that of other gregarines previously examined with the electron microscope. P. balani contains cell organelles comparable to those in the archigregarine Selenidium hollandei. These include mitochondria, Golgi complexes, granular and agranular endoplasmic reticulum, membrane-limited granules, vacuoles and myonemes. A comparison of the distribution and fine structure of these organelles with those of the above archigregarine was made. The possible function of the Golgi complex in assisting in producing the transverse septum is suggested. The surface of P. balani is composed of a surface-membrane complex different from that of S. hollandei, and similar to those of the eugregarines previously studied. The complex is composed of an outer plasmalemma, subjacent pentalaminar layer, and underlying, homogeneous, electron-dense layer. The pentalaminar layer is 140–170 Å thick and has a structure reminiscent of fused cell membranes seen in tight junctions, myelin figures and nerve myelin. The pentalaminar layer is derived from subsurface cisternae found in sporozoites. The underlying, homogeneous, electron-dense layer is 450–500 Å thick. Pores and caveolae are found as specializations of the pentalaminar and homogeneous electron-dense layers. Endoplasmic reticulum is closely associated with the caveolae. The location of the pentalaminar layer at the surface and the presence of pores and caveolae in its structure suggests some transport and/or segregation function.