ERYLUS ROTUNDUS. 295 



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

 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 ))rotruding parts of the sponge. In some cases, as for 

 instance in megarhabdu, 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 nun. broad, free from megascleres and flagellate 

 chambers, extentls 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 appear 

 to 1)6 isolated, are 100-150 /( 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 reimiants of the lumen of the strong!}' 

 contracted oscular tubes. 



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

 acanthtyhi-sters with not very numerous rays, small oxyastcrs with numerous 

 rays, and aspitlasters. In several forms also asters, resembling the acanthtvl- 

 asters in size and ray-numlier, but with conical raj^s, which become very slender 

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

 in var. megarhabdu, form A, are in all probability merely young stages of the 

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

 spicule fomi. In var. cidaris aster-like rhabd-clustens 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 arc in var. megarhabda, form A, up to 100 fi broad. In the other 

 forms most of the bundles are 10-40 /( thick. The Isroad bundles of var. mega- 

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

 drical. The isolated rhabds arc, 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 

 rhalxls is j^articularly well marketl in var. megarhabda, form A. 



The asterose rhabtl-clusters of var. cidaris are scattered in the choanosome. 



In var. megarhabda the armour is composed chiefly of oliliquely or radially 

 situated microrhabds, aspidasters being relatively scarce and confined to its 



