XXX THE VOYAGE OF H.M.S. CHALLENGER. 



The central capsule of the Acanthapja has hitherto been for the most part confounded with 

 that of the Spumellaeia, and no clear distinction has been drawn in this respect between the two 

 legions of the Porulosa. Hertwig, who in 1879 first discovered the remarkably different structure 

 of the Osculosa (Nassellaeia and Ph.-eodaeia), recognised no distinction between the structure of 

 the capsules in the Peeipylea and Actipylea (his Acanthometrea), and supposed that in both these 

 legions " very fine pores were evenly distributed in large numbers over the capsule-membrane " 

 {loc. cit, p. 106). I have, however, during the last few years convinced myself, by the careful com- 

 l^arative investigation of numerous Acanthaeia, that in this respect they are quite distinct from the 

 Spumellaeia (with perhaps the exception of the Astrolophida, which are nearly related to the 

 primitive Adissa). The number of pores in the Actipylea is usually very much smaller than in 

 the Peeipylea, and they are regularly arranged in groups. 



59. TJie Capsule-Openings of the Monopylea [or Nassellaria.) — The capsule-meml)rane 

 of the Monopylea always possesses a single large main- opening, an osculum, which lies 

 at the basal pole of the main axis, and is closed by a circular perforated lid (operculum 

 porosum). When seen from the surface this lid appears as a clearly defined porous area 

 (porochora or area porosa), and forms the horizontal base of a peculiar cone, which stands 

 vertically in the interior of the capsule and may be designated the " thread- cone " 

 (podoconus). The Nassellaria may hence be termed " Merotrypasta " or "Osculosa," 

 like the Cannopylea ; the structure and significance of the circular lid (operculum), 

 which closes the main-opening (osculum) is, however, quite diflferent in the two legions. 

 Whilst the lid of the Cannopylea (astropyle) is solid, traversed by radial ribs, and only 

 perforated in its centre by a short tube (proboscis), in the Monopylea the operculum 

 (porochora) is always perforated by numerous vertical fine pores, and is in connection 

 with the peculiar internal " pseudopodial cone" (podoconus, PL 51, figs. 5, 13 ; PI. 81, 

 fig. 16; PI. 91, fig. 5; PL 98, fig. 13). The pores are separated by small vertical, 

 highly refractive rods (opercular rhabdillse) ; these become intensely stained by carmine, 

 and are either evenly distributed over the surface of the porochora or arranged in definite 

 groups. The outer or distal end of each rod is rounded, sometimes thickened like a club 

 or split into lobes ; the inner or proximal end is usually pointed, and stands in connection 

 with a myophane thread of the podoconus (see § 79). The primary cii'cular form of the 

 porochora, in which the opercular rhabdillse are evenly distributed in a horizontal plane, 

 undergoes various secondary modifications in many NassellaPiIa. The triradial structure 

 of the skeleton, which characterises the majority of the legion, causes a splitting of the 

 base of the central capsule into three or four lobes ; tliis division also afi"ects the 

 porochora, which lies in the centre of the base, so that the rhabdillse become arranged in 

 three or four equal circles. If, however, the lobes of the central capsule become larger 

 and protrude through the three or four collar pores of the cortinar septum, the central 

 porochora may separate entirely into three or four elongated tracts, which lie on the axial 

 side of the magnified lobes ; the rhabdillse are then arranged over the whole surface of 



