B. The violin-shaped concretions of _Thalassosphæra bifurca_ have been figured in my Monograph (pp. 80, 261, Taf. xii. fig. 1). 76. _The Intracapsular Xanthellæ._--The xanthellæ, zooxanthellæ, or symbiotic "yellow cells" are found within the central capsule only in the ACANTHARIA, whilst in other Radiolaria they only occur in the extracapsulum. They are most frequent in the #Acanthometra#, rarer in the #Acanthophracta#, but even in the former they are often wanting. Their number is very variable, but usually small, from ten to thirty in one capsule. They lie for the most part immediately below the capsule membrane, in the cortical layer of the endoplasm. The form of the yellow cells is either spherical or ellipsoidal, often also spheroidal or even lentiform. The diameter varies from 0.01 to 0.03 mm. They possess a distinct membrane and an excentric nucleus, and contain numerous yellow pigment-granules in the endoplasm. This yellow pigment dissolves in mineral acids to form a green fluid, and in other respects also behaves somewhat differently from the yellow pigment in the extracapsular yellow cells of the SPUMELLARIA and NASSELLARIA. In both cases, however, the xanthellæ are not integral portions of the organism, but unicellular algae, living as parasites or symbiontes in the body. A. The yellow cells in the central capsule of the ACANTHARIA were first observed by Joh. Müller (L. N. 12, pp. 14, 47). In my Monograph I described them at greater length, and indicated their differences from the extracapsular yellow cells of other Radiolaria (L. N. 16, pp. 77, 86). Since then, R. Hertwig has demonstrated their cellular nature (L. N. 33, pp. 12, 113), and still more recently {xlvi}Brandt has given further accurate information regarding their occurrence, constitution, and physiological significance (L. N. 39, ii. Art., p. 235, figs. 62-73).