ERYLUS ROTUNDUS. 297 



to within a short chstance of the outer surface. In one of the specimens of var. 

 megarhabda, form A, I failed to find any of these asters in situ in the sections. 



The aspidasters take part in the formation of the cortical armour and are 

 also found scattered in the choanosome. In vars. iypica and cidaris the greater 

 part of the armour is composed of these spicules, which are here absent only in 

 the mantles surrounding the pore-canals. Apart from these mantles, the 

 proximal (internal) part of the aimour in these varieties consists entirely of 

 aspidasters. In the distal (external) part of the armour microrhabds are usually 

 added to the aspidasters, and sometimes the outermost part of the armour con- 

 sists entirely of microrhabds. Most of the aspidasters of the armour are situated 

 paratangentially. Under exix)sed tracts of the surface they form many layers, 

 under the sheltered parts of it which limit the cavities, extending into the inte- 

 rior, only few layers or only a single layer. In var. megarhabda the aspidasters 

 form only a small part of the armour and arc here confined to its distal (external) 

 part. Young and also adult aspidasters are usually found scattered in the 

 choanosome. Here they generally lie in cavities of the ground-substance, as 

 long and broad but much wider (thicker) than the aspidasters, so that the mar- 

 gins of the aspidasters are in contact with the ground-substance, while their 

 faces are separated from it by apparently empty spaces. Seen en face the aspid- 

 asters consequently appear to fill these cavities completely, seen in profile they 

 appear as narrow bars occupjdng the long axis of the oval cavities. The empty 

 spaces at their sides may of course have been produced by the shrinkage of the 

 tissue, during the preservation of the sponges in alcohol, but they may also be 

 natural, and in this case occupied possibly by some liquid, rich in silica secreted 

 from the surrounding tissue, from which the cell or cells building the aspidaster 

 draw their su]iply. 



The rhabd megascleres (Plate 5, figs. 11-23, 26-28a) are for the most part 

 simple amphioxes or ampliistrongyles, curved uniformly, or in the middle 

 more strongly than near the ends. Occasionally style (Plate 5, fig. 17c) and 

 angularly bent or branched derivates of these rhabds have been observed. The 

 ordinary amphioxes and amphistrongyles are 310-650 /i long, and 6-15 fi thick. 

 Of the amphioxes and amphistrongyles occurring together in the same speci- 

 men, the former are on the whole longer than the latter. In the vars. mega- 

 rhabda and c/d«/-/s nearly all the rhabds are sharp-pointed amphioxes (Plate 5, 

 figs. 19-23, 26-28a), blunt amphioxes and amphistrongyles (Plate 5, fig. 18) 

 being rare. In the forms B and C of var. typica, sharp-pointed ampliioxes 

 (Plate 6, figs. 14, 15, 17a) also preponderate; in the forms A and D of this 

 variety, on the other hand, the blunt rhabds (amphistrongyles) (Plate 5, figs. 



