296 



ERYLUS ROTUNDUS. 



superficial part. In this variety numerous microrhabds also occur scattered 

 in the choanosome. In vars. typica and cidaris, on the other hand, the armour 

 is chiefly composed of aspidasters, and here the microrhabds are confined to its 

 superficial part, except in the vicinity of the pores, around which they form 

 mantles, extending right through the whole armour. Sometimes the superficial 

 microrhabd-bearing part of the armour-layer is stout and well developed, and 

 then it consists of an outer zone composed of paratangial microrhabds and an 

 inner zone of oblique and vertical (radial) microrhabds, lying above and be- 

 tween the outermost aspidasters. Often, however, this microrhabd-bearing 

 outer armour-layer is insignificant, and then composed only of relatively few, 

 mostly oblique microrhabds. It is possible that the superficial parts of the 

 sponges presenting this appearance have been rubbed off. The majority of the 

 microrhabds in the pore-canal mantles are situated so that one of their ends 

 points obliquely upward towards the centre of the })ore. In some forms of 

 var. typica, particularly in form B, a fair number of microrhabds were also found 

 scattered in the choanosome. 



The cladomes of the triaenes extend paratangentially just below the cortex 

 or within its lower, fibrous layer; their rhabdomes are directed radially inward. 

 The triaenes occupy the interporal spaces and in some forms, as for instance 

 in var. typica, form A (Plate 6, fig. 27a), form well-defined groups, in which a 

 number of triaenes lie close together at the points of intersection of the inter- 

 poral zones. 



The acanthtylasters and their oxyaster-like young are scattered through- 

 out the choanosome. They are most abundant in one of the specimens of 

 var. megarhabda, form A. In some forms, as for instance in var. typica, form B, 

 they are very much scarcer in the subcortical region than in the interior of the 

 choanosome. A great many acanthtylasters lie in the walls of the choanosomal 

 canals. Some of the rays of these usually protrude into the canal-lumen. In 

 the walls of the remnants of the contracted oscular tubes of var. cidaris, above 

 referred to, the acanthtylasters stand particularly close together and here form a 

 veritable pavement. This local acanthtylaster-density is doubtless due to 

 the contraction of the surface on which, when normally extended, they are 

 probably distributed in the ordinary, not particularly dense manner. 



The small oxyasters with numerous rays are confined to the roofs of the 

 subcortical cavities and the walls of the pore-canals. In the choanosome they 

 appear to be entirely absent. In the walls of the pore-canals, where they are 

 most numerous and sometimes form quite a dense layer, they extend right up 



