ERYLUS ROTUNDUS. 



295 



to be of much greater width than the afferents. In both forms of var. 7nega- 

 rhabda and in some specimens of the forms A and B of var. typica on the other 

 hand, larger oscules, up to 1.4 mm. wide in the former, and up to 2 mm. wide 

 in the latter variety, have been observed. These larger oscules usually lie on 

 or near the summit of protruding parts of the sponge. In some cases, as for 

 instance in megarhabda, form B, large oscular tubes, up to 2.4 mm. in diameter, 

 lead up to the oscules. In other cases, as for instance in var. cidaris, a tract of 

 transparent tissue, about 1.7 mm. broad, free from megascleres and flagellate 

 chambers, extends from each of the here strongly contracted or evenly closed 

 oscules, down into the interior of the sponge. In the axis of this tract a row of 

 small cavities is observed. These cavities, which in the radial sections api^ear 

 to be isolated, are 100-150 /j. broad, up to 350 /< long, and situated close together. 

 Distally, towards the contracted oscule, they become smaller and scarcer. I 

 consider these rows of cavities as the remnants of the lumen of the strongly 

 contracted oscular tubes. 



The skeleton consists of rhabd megascleres, microrhabds, triaenes, large 

 acanthtylasters with not very numerous rays, small oxyasters with numerous 

 rays, and aspidasters. In several forms also asters, resembling the acanthtyl- 

 asters in size and ray-number, but with conical rays, which become very slender 

 distally, have been observed. These spicules, which are particularly numerous 

 in var. megarhabda, form A, are in all probability merely 3^oung stages of the 

 ordinary acanthtylasters. I shall not therefore deal with them as a special 

 spicule form. In var. cidaris aster-like rhabd-clusters have been observed. 



Some of the rhabd megascleres are isolated, others form more or less undu- 

 lating bundles (Plate 6, fig. 25a), which traverse the internal parts of the choano- 

 some in a radial or, in the digitate and lobose processes, longitudinal direction, 

 and, on nearing the cortex, tend to assume a position vertical to the surface. 

 These bundles are in var. megarhabda, form A, up to 100 /« broad. In the other 

 forms most of the bundles are 10 40 /< thick. The l)road bundles of var. mega- 

 rhabda, form A, appear to be flattened, band shaped; the narrow ones are cylin- 

 drical. The isolated rhabds are, in the interior, quite irregularly scattered; 

 near the surface, just below the cortex, most of them usually assume a position 

 more or less vertical to the surface. This radial arrangement of the subcortical 

 rhabds is particularly well marked in var. megarhabda, form A. 



The asterose rhabd-clusters of var. cidaris are scattered in the choanosonie. 



In var. megarhabda the armour is composed chiefly of oblic^uely or radially 

 situated microrhabds, aspidasters being relatively scarce and confined to its 



