REPORT ON THE RADIOLARIA. 905 



The Central Capsule of the Plectoidea constantly exhibits the peculiar 

 characters of the MONOPYLEA or NASSELLARIA. It is commonly ovate, more rarely 

 ellipsoidal or even spherical, sometimes conical or lentelliptical. The lower or basal 

 pole of its vertical main axis constantly exhibits the characteristic " porochora " (or the 

 area porosa) of the MONOPYLEA, and upon this rests the peculiar " podoconus " (or the 

 pseudopodial-cone) of this legion. On this porochora the central capsule is in 

 immediate connection with the central point of the skeleton, or the horizontal common 

 central rod, from which the radial spines arise. The endoplasm, or the protoplasm of 

 the central capsule (besides the podoconus), contains commonly one large alveole or 

 several small vacuoles, and often pigment-granules. The nucleus is large, spherical or 

 ovate, and exhibits the same character as in all the other MONOPYLEA ; it encloses 

 usually a single nucleolus. 



The position of the central capsule and its topographical relation to the skeleton 

 offers in the different Plectoidea some important and as yet unsolved problems, 

 which can be answered only by fresh and accurate observations on living specimens. In 

 Triplagia, and Triplecta, where the triangular skeleton lies in a horizontal plane, 

 the vertical main axis of the central capsule is perpendicular to the central point of that 

 supporting triangle. In Plagiacantha and Plectophora, where the three radial spines 

 correspond to the edges of a flat pyramid, the capsule is enclosed in the pyramidal 

 space of the latter, its basal pole touching the apex ; therefore in the normal position of 

 the body the three divergent rays are directed upwards. In Tetraplagia and 

 Tetraplecta probably the same position is retained, and therefore the fourth free 

 spine, here developed, is probably directed vertically downwards. In Plagoniscus and 

 Plectaniscus, however, and, moreover, in the closely allied Plagiocarpa and the corres- 

 ponding Periplecta (PI. 91, figs. 5, 10) the position of the central capsule, relative to 

 the skeleton, seems to be inverse, and to agree with that of the Stephoidea 

 (Cortina, Cortiniscus, &c.) and the Cyrtoidea (Pteroscenium, Clathrocorys, &c.); 

 the three divergent spines are here directed downwards (as basal feet), whilst the 

 opposite fourth spine is vertically directed upwards (as an apical horn); the capsule 

 rests here upon the tripod, which lies below it, and is inclined with its dorsal face to the 

 apical spine. In the majority of the other Plectoidea the position of the central 

 capsule and its relation to the skeleton are not yet sufficiently observed, and require 

 further accurate researches. Its position seems to be very different in the several genera. 

 The capsule is never perforated by parts of the skeleton; this latter is constantly 

 extra-capsular. 



The physiological value of the skeleton, with regard to the central capsule, is 

 different in the Plagonida and Plectanida; in the former it supports, in the latter it 

 encloses the capsule like a shell. In the Nassellida, where no skeleton is developed, the 

 central capsule is quite free and naked, enveloped only by the calymma. 



(ZOOL. CHALL. EXP. PART XL. 1885.) Rr 114 



